CSS Color Module Level 4

CSS Color Module Level 4
CSS Color Module Level 4
W3C Candidate Recommendation Draft
14 April 2026
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W3C
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L. David Baron
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Abstract
This specification describes CSS

values, and properties for foreground color and group opacity. It also describes how colors are interpolated, and how to gamut map colors.
CSS
is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, etc.
Status of this document
This section describes the status of this document at the time of its publication.
A list of current W3C publications
and the latest revision of this technical report
can be found in the
W3C standards and drafts index.
This document was published
by the
CSS Working Group
as a
Candidate Recommendation Draft
using the
Recommendation
track
Publication as a Candidate Recommendation
does not imply endorsement by
W3C
and its Members.
A Candidate Recommendation Draft
integrates changes from the previous Candidate Recommendation
that the Working Group intends to include in a subsequent Candidate Recommendation Snapshot.
This is a draft document
and may be updated, replaced
or obsoleted by other documents at any time.
It is inappropriate to cite this document as other than a work in progress.
Please send feedback
by
filing issues in GitHub
(preferred),
including the spec code “css-color” in the title, like this:
“[css-color]
…summary of comment…
”.
All issues and comments are
archived
Alternately, feedback can be sent to the (
archived
) public mailing list
www-style@w3.org
This document is governed by the
18 August 2025 W3C Process Document
This document was produced by a group operating under the
W3C Patent Policy
W3C maintains a
public list of any patent disclosures
made in connection with the deliverables of the group;
that page also includes instructions for disclosing a patent.
An individual who has actual knowledge of a patent that the individual believes
contains
Essential Claim(s)
must disclose the information in accordance with
section 6 of the W3C Patent Policy
1.
Introduction
This section is not normative.
Tests
This section is not normative, it does not need tests.
This module describes CSS properties which
allow authors to specify
the foreground color and opacity of the text content of an element.
This module also describes in detail the CSS

value type.
It not only defines the color-related properties and values that
already exist in
CSS1
CSS2
and
CSS Color 3
but also defines new properties and values.
In particular, it allows specifying colors
in other
color spaces
than sRGB;
previously, the more saturated colors outside the sRGB gamut
could not be used in CSS
even if the display device supported them.
draft implementation report
is available.
1.1.
Value Definitions
This specification follows the
CSS property definition conventions
from
[CSS2]
using the
value definition syntax
from
[CSS-VALUES-3]
Value types not defined in this specification are defined in CSS Values & Units
[CSS-VALUES-3]
Combination with other CSS modules may expand the definitions of these value types.
In addition to the property-specific values listed in their definitions,
all properties defined in this specification
also accept the
CSS-wide keywords
as their property value.
For readability they have not been repeated explicitly.
2.
Color Terminology
Tests
This section provides definitions used later, it does not need tests.
color
is a definition (numeric or textual)
of the human visual perception of a light
or a physical object illuminated with light.
The objective study of human color perception is termed
colorimetry
The color of a physical object
depends on how much light it reflects
at each visible wavelength,
plus the actual color of the light illuminating it
(again, the amount of light at each wavelength).
It is measured by a
spectrophotometer
The color of something that emits light
(including colors on a computer screen)
depends on how much light it emits
at each visible wavelength.
It is measured by a
spectroradiometer
If two objects have different
spectra
but still produce the same physical sensation,
we say they have the same color.
We can calculate whether two colors are the same
by converting the spectra to CIE XYZ
(three numbers).
For example a green leaf, a photograph of that leaf
displayed on a computer screen, and a print of that photograph,
are all producing a green sensation by different means.
If the screen and the printer are
calibrated
the green in the leaf, and the photo, and the print will look the same.
color space
is an organization of colors
with respect to an underlying
colorimetric
model,
such that there is a clear, objectively-measurable meaning
for any color in that color space.
This also means that the same color can be expressed in multiple color spaces,
or transformed from one color space to another,
while still looking the same.
A leaf is measured
with a spectrophotometer
and found to have the color
lch(51.2345% 21.2 130)
which is
lab(51.2345% -13.6271 16.2401).
This same color could be expressed in various color spaces:
color
sRGB
0.41587
0.503670
0.36664
);
color
display-p3
0.43313
0.50108
0.37950
);
color
a98-rgb
0.44091
0.49971
0.37408
);
color
prophoto-rgb
0.36589
0.41717
0.31333
);
color
rec2020
0.6295
0.9657
0.3633
);
An
additive color space
means that the coordinate system is linear in light intensity.
The
CIE
XYZ color space is an additive color space.
The Y component of XYZ is the
luminance
, the light intensity per unit area,
or 'how bright it is'. Luminance is measured in candelas per square meter.
cd/m², also called
nits
In an additive color space, calculations can be done
to
accurately predict
color mixing. Most RGB spaces
are not additive, because the components are
gamma encoded
. Undoing this gamma encoding
produces linear-light values.
For example, if a light fixture contains two identical colored lights,
and only one is switched on,
and the color is measured to be
color(xyz 0.13 0.12 0.04),
then the color when both are switched on will be exactly twice that,
color(xyz 0.26 0.24 0.08).
If we have two differently colored spotlights shining on a stage,
and one has the measured value
color(xyz 0.15 0.24 0.17)
while the other is
color(xyz 0.11 0.06 0.06)
then we can accurately predict that if the colored beams are made to overlap,
the color of the mixture will be the sum
of the XYZ component values, or
color(xyz 0.26 0.30 0.23).
chromaticity
is a color measurement
where the lightness component has been factored out.
From the identical lights example above,
the
u',v'
chromaticity with one light is
(0.2537, 0.5268)
and the chromaticity is the same with both lights
(they are the same color, it is just brighter).
Chromaticities are additive,
so they accurately predict
the chromaticity (but not the resulting lightness)
of a mixture.
Being two-dimensional, chromaticity is easily represented
on a
chromaticity diagram
to predict the chromaticity of a color mixture.
Any two colors can be mixed, and the resulting colors
will lie on the line joining them on the diagram.
Three colors form a plane, and the resulting colors
will lie in the triangle they form on the diagram.
A chromaticity diagram showing
(in solid colors) the
display-p3
color space
and for comparison
(faded) the
sRGB
color space.
The white point (D65) is also shown.
Thus, once linearized, RGB color spaces are additive,
and their gamut is defined
by the chromaticities of the red, green and blue primaries,
plus the chromaticity of the
white point
(the color formed by all three primaries at full intensity).
Most color spaces use one of a few
daylight-simulating
white points
which are named by the correlated color temperature (CCT)
[Understanding_CCT]
of the corresponding black-body radiator.
For example,
D65
is a daylight whitepoint
corresponding to a correlated color temperature
of 6500 Kelvin
(actually 6504,
because the value of Plank’s constant has changed
since the color was originally defined).
To avoid cumulative round-trip errors,
it is important that the identical chromaticity values
are used consistently,
at all places in a calculation. Thus,
for maximum compatibility,
for this specification,
the following two standard
daylight-simulating
white points
are defined:
Name
CCT
D50
0.345700
0.358500
5003K
D65
0.312700
0.329000
6504K
When the measured physical characteristics
(such as the
chromaticities
of the primary colors it uses,
or the colors produced in response to a given set of inputs)
of a
color space
or a color-producing device are known,
it is said to be
characterized
If in addition adjustments have been made so that a device meets calibration targets
such as white point, neutrality of greys, predictability and consistency of tone response,
then it is said to be
calibrated
Real physical devices cannot yet produce every possible color that the human eye can see.
The range of colors that a given device can produce is termed the
gamut
(not to be confused with gamma)
Devices with a limited gamut cannot produce very saturated colors,
like those found in a rainbow.
A top-down view of three gamuts, plotted in Oklab with the positive a-axis towards the right and the positive b-axis towards the top; looking down the l-axis so white and neutrals are in the center. The largest of the three gamuts is ITU Rec BT.2020; the medium-sized one is Display P3, and the smallest is sRGB. Rendering by Alexey Ardov.
The gamuts of different
color space
s may be compared
by looking at the volume (in cubic Lab units) of colors that can be expressed.
The following table examines the
predefined
color spaces available in CSS.
color space
Volume (million Lab units)
sRGB
0.820
display-p3
1.233
a98-rgb
1.310
prophoto-rgb
2.896
rec2020
2.042
A color in CSS is either an
invalid color
as described below for each syntactic form,
or a
valid color
Any color which is not an
invalid color
is a
valid color
A color may be a
valid color
but still be outside the range of colors
that can be produced by an output device
(a screen, projector, or printer)
It is said to be
out of gamut
Each
valid color
is either
in-gamut
for a particular output device (screen, or printer)
or it is
out of gamut
For example, given a screen which covers 100% of the display-p3 color space,
but no more, the following color is out of gamut:
color
prophoto-rgb
0.88
0.45
0.10
because, expressed in display-p3,
one or more coordinates are either greater that 1.0 or less than 0.0:
color
display-p3
1.0844
0.43
0.1
This color is valid,
and could, for example, be used as a gradient stop,
but would need to be
CSS gamut mapped
for display,
producing a similar-looking but lower chroma (less saturated) color.
3.
Applying Color in CSS
3.1.
Accessibility and Conveying Information By Color
Tests
This section provides authoring guidance, it does not need tests.
Although colors can add significant information to documents
and make them more readable,
color by itself should not be the sole means to convey important information.
Authors should consider the W3C Web Content Accessibility Guidelines
[WCAG21]
when using color in their documents.
1.4.1 Use of Color:
Color is not used as the only visual means of conveying information,
indicating an action, prompting a response, or distinguishing a visual element
3.2.
Foreground Color: the
color
property
Name:
color
Value:

Initial:
CanvasText
Applies to:
all elements and text
Inherited:
yes
Percentages:
N/A
Computed value:
computed color, see
resolving color values
Canonical order:
per grammar
Animation type:
by computed value type
Tests
color-001.html
(live test)
(source)
color-002.html
(live test)
(source)
color-003.html
(live test)
(source)
inheritance.html
(live test)
(source)
color-interpolation.html
(live test)
(source)
color-initial-canvastext.html
(live test)
(source)
color-valid.html
(live test)
(source)
color-invalid.html
(live test)
(source)
This property specifies the primary foreground color of the element.
This is used as the fill color of its text content,
and in addition specifies the
used value
that
currentcolor
resolves to,
which allows indirect references to this foreground color
and affects the initial values of various other color properties
such as
border-color
and
text-emphasis-color

Sets the primary foreground color to the specified

The

type provides multiple ways to syntactically specify a given color.
For example, the following declarations all specify the sRGB color “lime”:
em
color
lime
/* color keyword */
em
color
rgb
255
);
/* RGB range 0-255 */
em
color
rgb
100
);
/* RGB range 0%-100% */
em
color
color
sRGB
);
/* sRGB range 0.0-1.0 */
When applied to text, this property, including its alpha component,
has no effect on “color glyphs” (such as the emoji in some fonts),
which are colored by a built-in palette.
However, some colored fonts are able to refer to a contextual “foreground color”,
such as by palette entry
0xFFFF
in the
COLR
table of OpenType,
or by the
context-fill
value in SVG-in-OpenType.
In such cases, the foreground color is set by this property,
identical to how it sets the
currentcolor
value.
3.3.
Transparency: the
opacity
property
Opacity can be thought of as a postprocessing operation.
Conceptually, after the element (including its descendants) is rendered into an RGBA offscreen image,
the opacity setting specifies how to blend the offscreen rendering into
the current composite rendering.
See
simple alpha compositing
for details.
Name:
opacity
Value:

Initial:
Applies to:
all elements
Inherited:
no
Percentages:
map to the range [0,1]
Computed value:
specified number, clamped to the range [0,1]
Canonical order:
per grammar
Animation type:
by computed value type
Tests
clip-opacity-out-of-flow.html
(live test)
(source)
t32-opacity-basic-0.0-a.xht
(live test)
(source)
t32-opacity-basic-0.6-a.xht
(live test)
(source)
t32-opacity-basic-1.0-a.xht
(live test)
(source)
t32-opacity-clamping-0.0-b.xht
(live test)
(source)
t32-opacity-clamping-1.0-b.xht
(live test)
(source)
t32-opacity-offscreen-b.xht
(live test)
(source)
t32-opacity-offscreen-multiple-boxes-1-c.xht
(live test)
(source)
t32-opacity-offscreen-multiple-boxes-2-c.xht
(live test)
(source)
t32-opacity-offscreen-with-alpha-c.xht
(live test)
(source)
t32-opacity-zorder-c.xht
(live test)
(source)
opacity-computed.html
(live test)
(source)
opacity-valid.html
(live test)
(source)
opacity-invalid.html
(live test)
(source)
composited-filters-under-opacity.html
(live test)
(source)
filters-under-will-change-opacity.html
(live test)
(source)
color-composition.html
(live test)
(source)
opacity-interpolation.html
(live test)
(source)
canvas-change-opacity.html
(live test)
(source)
opacity-animation-ending-correctly-001.html
(live test)
(source)
opacity-animation-ending-correctly-002.html
(live test)
(source)

The opacity to be applied to the element.
The resulting opacity is applied to the entire element,
rather than a particular color.
Opacity values outside the range [0,1] are not invalid,
and are preserved in specified values,
but are clamped to the range [0, 1]
in computed values.
Tests
inline-opacity-float-child.html
(live test)
(source)
Opacity in CSS is represented using the

syntax,
for example in the
opacity
property.



Represented as a

, the useful range of the value is
(representing full transparency)
to
(representing full opacity).
It can also be written as a

which
computes to
the equivalent

0%
to
100%
to
).
The
opacity
property applies the specified opacity to the element
as a whole
including its contents,
rather than applying it to each descendant individually.
This means that, for example,
an opaque child occluding part of the element’s background will continue to do so even when
opacity
is less than 1,
but the element and child as a whole will show the underlying page through themselves.
It also means that the glyphs
corresponding to all characters in the element
are treated
as a whole
any overlapping portions do not increase the opacity.
Tests
opacity-overlapping-letters.html
(live test)
(source)
Correct and incorrect rendering of text
with an
opacity
value of less than one,
whose glyphs overlap.
If separate opacity for each glyph is desired,
it can be achieved by using a color value
which includes alpha,
rather than setting the
opacity
property.
If a box has
opacity
less than 1,
it forms a
stacking context
for its children.
(This prevents its contents from interleaving in the z-axis
with content outside it.)
Tests
body-opacity-0-to-1-stacking-context.html
(live test)
(source)
Furthermore, if the
z-index
property applies to the box,
the
auto
value is treated as
for the element;
it is otherwise painted on the same layer within its parent stacking context
as positioned elements with stack level 0
(as if it were a positioned element with
z-index:0
).
See
section 9.9
and
Appendix E
of
[CSS2]
for more information on stacking contexts.
These rules about z-order do not apply to SVG elements,
since SVG has its own
rendering model
[SVG11]
, Chapter 3).
The value of the
opacity
property
does
not
affect hit testing.
3.4.
Color Space of Tagged Images
An
tagged image
is an image
that is explicitly assigned a color profile,
as defined by the image format.
This is usually done by including an
International Color Consortium (ICC) profile
[ICC]
For example JPEG
[JPEG]
, PNG
[PNG]
and TIFF
[TIFF]
all specify a means to embed an ICC profile.
Image formats may also use other, equivalent methods, often for brevity.
For example, PNG specifies a means (the
sRGB chunk
to explicitly tag an image as being in the sRGB color space,
without including the sRGB ICC profile.
Similarly, PNG specifies a compact means
(the
cICP chunk
to explicitly tag an image as being one of various SDR or HDR color spaces,
such as Display P3 or BT.2100 HLG,
without including an ICC profile.
Tagged RGB images,
and tagged images using a transformation of RGB such as YCbCr,
if the color profile or other identifying information is valid,
must be treated as being in the specified color space.
Tests
tagged-images-001.html
(live test)
(source)
tagged-images-002.html
(live test)
(source)
tagged-images-003.html
(live test)
(source)
tagged-images-004.html
(live test)
(source)
cicp-chunk.html
(live test)
(source)
fDAT-inherits-cICP.html
(live test)
(source)
For example, when a browser running on a system with a Display P3 monitor
displays an JPEG image
tagged as being in the ITU Rec BT.2020
[Rec.2020]
color space, it must convert the colors
from ITU Rec BT.2020 to Display P3
so that they display correctly.
It must not treat the ITU Rec BT.2020 values
as if they were Display P3 values, which would produce incorrect colors.
If the color profile or other identifying information is invalid, the image is treated as described for
untagged images
3.5.
Color Spaces of Untagged Colors
For compatibility, colors specified in HTML,
and
untagged images
must be treated
as being in the sRGB color space (
[SRGB]
unless otherwise specified.
Tests
untagged-images-001.html
(live test)
(source)
An
untagged image
is an image that is not explicitly assigned a color profile, as defined by the image format.
This rule does not apply to untagged videos, since
untagged video
should be presumed to be in an ITU-defined color space.
At below 720p, it is Recommendation ITU-R BT.601
[ITU-R-BT.601]
At 720p, it is SMPTE ST 296 (same colorimetry as 709)
[SMPTE296]
At 1080p, it is Recommendation ITU-R BT.709
[ITU-R-BT.709]
At 4k (UHDTV) and above, it is ITU-R BT.2020
[Rec.2020]
for SDR video
4.
Representing Colors: the

type
Tests
This section describes a type, it is primarily tested where that type is used.
Colors in CSS are represented as a list of color components,
also sometimes called “channels”,
representing axises in the color space.
Each component has a minimum and maximum value,
and can take any value between those two.
Additionally, every color is accompanied by
an
alpha component
indicating how transparent it is,
and thus how much of the backdrop one can see through the color.
CSS has several syntaxes for specifying color values:
the sRGB
hex color notation
which represents the RGB and alpha components in hexadecimal notation
the various
color functions
which can represent colors using a variety of color spaces and coordinate systems
the constant
named color
keywords
the variable

keywords and
currentColor
keyword.
The
color functions
use CSS
functional notation
to represent colors in a variety of
color spaces
by specifying their component coordinates.
Some of these use a
cylindrical polar color
model,
specifying color by a

angle,
a central axis representing lightness
(black-to-white),
and a radius representing saturation or chroma
(how far the color is from a neutral grey).
The others use a
rectangular orthogonal color
model,
specifying color using three
orthogonal component axes.
The
color functions
available in Level 4 are
rgb()
and its
rgba()
alias,
which (like the
hex color notation
) specify sRGB colors directly
by their red/green/blue/alpha components.
hsl()
and its
hsla()
alias,
which specify sRGB colors
by hue, saturation, and lightness
using the
HSL
cylindrical coordinate model.
hwb()
which specifies an sRGB color
by hue, whiteness, and blackness
using the
HWB
cylindrical coordinate model.
lab()
which specifies a CIELAB color
by CIE Lightness and its a- and b-axis hue coordinates
(red/green-ness, and yellow/blue-ness)
using the
CIE LAB rectangular coordinate model
lch()
which specifies a CIELAB color
by CIE Lightness, Chroma, and hue
using the
CIE LCH cylindrical coordinate model
oklab()
which specifies an Oklab color
by Oklab Lightness and its a- and b-axis hue coordinates
(red/green-ness, and yellow/blue-ness)
using the
Oklab
rectangular coordinate model.
oklch()
which specifies an Oklab color
by Oklab Lightness, Chroma, and hue
using the
OkLCh
cylindrical coordinate model.
color()
which allows specifying colors in a variety of color spaces
including
sRGB
Linear-Light sRGB
Display P3
Linear-Light Display P3
A98 RGB
ProPhoto RGB
ITU-R BT.2020-2
and
CIE XYZ
For easy reference in other specifications,
opaque black
is defined as the color
rgb(0 0 0 / 100%)
transparent black
is the same color,
but fully transparent—​i.e.
rgb(0 0 0 / 0%)
Tests
color-computed-named-color.html
(live test)
(source)
color-computed.html
(live test)
(source)
color-valid.html
(live test)
(source)
4.1.
The

syntax
Tests
This section provides definitions used later, it does not need tests.
Colors in CSS are represented by the

type:
=

currentColor





transparent

rgb
()
rgba
()
hsl
()
hsla
()
hwb
()
lab
()
lch
()
oklab
()
oklch
()
color
()
An
absolute color
is a

whose computed value
has an absolute, colorimetric interpretation.
This means that the value is not:
currentColor
(which depends on the value of the
color
property)

(which depends on the color mode)
The colors that
resolve to sRGB
are:
hex
colors
rgb()
and
rgba()
values
hsl()
and
hsla()
values
hwb()
values
named
colors
The functions that
support legacy color syntax
are:
rgb()
and
rgba()
hsl()
and
hsla()
The




, and

color functions
are
cylindrical polar color
representations using a

angle;
the other
color functions
use
rectangular orthogonal color
representations.
4.1.1.
Modern (Space-separated) Color Function Syntax
All of the
absolute color
functional forms
first defined in this specification
use the
modern color syntax
meaning:
color components are separated by whitespace
the optional alpha term is separated by a solidus ("/")
minimum required precision
when serializing
is defined,
and may be greater than 8 bits per component
the
none
value is allowed, to represent
missing components
components using

and

may be freely mixed
The following represents a saturated sRGB red that is 50% opaque:
rgb
100
50
4.1.2.
Legacy (Comma-separated) Color Function Syntax
For Web compatibility,
the syntactic forms
of
rgb()
rgba()
hsl()
, and
hsla()
(those defined in earlier specifications)
also support a
legacy color syntax
which has the following differences:
color components are separated by commas
(optionally preceded and/or followed by whitespace)
non-opaque forms use a separate notation
(for example
hsla()
rather than
hsl()
and the alpha term is separated by commas
(optionally preceded and/or followed by whitespace)
minimum required precision is lower, 8 bits per component
the
none
value is not allowed
color components must be specified using either all-

or all-

, they can not be mixed.
The following represents a saturated sRGB red that is 50% opaque:
rgba
100
0.5
For the
color functions
introduced
in this or subsequent levels,
where there is no Web compatibility issue,
the
legacy color syntax
is invalid.
4.2.
Representing Transparency in Colors: the

syntax
Tests
This section provides definitions used later, it does not need tests.



Unless otherwise specified,
an

component of a color defaults to
100%
when omitted.
Values outside the range [0,1] are not invalid,
but are clamped to that range at parsed-value time.
4.3.
Representing Cylindrical-coordinate Hues: the

syntax
Tests
This section provides definitions used later, it does not need tests.
Hue is represented as an angle of the color circle
(the rainbow, twisted around into a circle, and with purple added between violet and red).



Because this value is so often given in degrees,
the argument can also be given as a number,
which is interpreted as a number of degrees
and is the
canonical unit
This number is normalized
to the range [0,360).
For example, in
hsl(-540 0 0)
or
hsl(540 0 0)
the

component is normalized to 180 degrees.
In
hsl(360 0 0)
the

component is normalized to 0 degrees.
In
hsl(calc(-infinity) 0 0)
or
hsl(calc(infinity) 0 0)
the

component is again normalized to 0 degrees.
Note:
The angles and spacing
corresponding to particular hues
depend on the color space.
For example, in HSL and HWB, which use the sRGB color space,
sRGB green is 120 degrees.
In LCH, sRGB green is 134.39 degrees,
display-p3 green is 136.01 degrees,
a98-rgb green is 145.97 degrees
and prophoto-rgb green is 141.04 degrees
(because these are all different shades of green).

components are the most common components to become
powerless
any color sufficiently close to the central achromatic axis
will have a
powerless
hue component.
4.4.
“Missing” Color Components and the
none
Keyword
In certain cases,
a color can have one or more
missing color components
In this specification,
this happens automatically due to
hue-based interpolation
for some colors (such as
white
);
other specifications can define additional situations
in which components are automatically missing.
It can also be specified explicitly,
by providing the keyword
none
for a component in a color function.
All color functions
(with the exception of those using the
legacy color syntax
allow any of their components to be specified as
none
This should be done with care,
and only when the particular effect of doing so is desired.
Tests
color-computed-color-function.html
(live test)
(source)
color-computed-hsl.html
(live test)
(source)
color-computed-hwb.html
(live test)
(source)
color-computed-lab.html
(live test)
(source)
color-computed-relative-color.html
(live test)
(source)
color-computed-rgb.html
(live test)
(source)
color-invalid-hsl.html
(live test)
(source)
color-invalid-rgb.html
(live test)
(source)
color-valid-color-function.html
(live test)
(source)
color-valid-color-mix-function.html
(live test)
(source)
color-valid-hsl.html
(live test)
(source)
color-valid-hwb.html
(live test)
(source)
color-valid-lab.html
(live test)
(source)
color-valid-relative-color.html
(live test)
(source)
color-valid-rgb.html
(live test)
(source)
For handling of
missing components
in
situations which combine two colors,
such as color interpolation,
see
§ 13.2 Interpolating with Missing Components
For all other purposes, a
missing component
behaves as a zero value,
in the appropriate unit for that component:
0%
, or
0deg
This includes rendering the color directly,
converting it to another color space,
performing computations on the color component values,
etc.
If a color with a
missing component
is serialized
or otherwise presented directly to an author,
then for
legacy color syntax
it represents that component as a zero value;
otherwise,
it represents that component as being the
none
keyword.
A missing hue is common when interpolating in cylindrical color spaces.
For example, using the
color-mix()
function specified in
[CSS-COLOR-5]
one could write
color-mix(in hsl, white 30%, green 70%)
Since
white
is an achromatic color,
it has a
missing
hue when expressed in
hsl()
(effectively
hsl(none 0% 100%))
since
any
hue will produce the same color)
which means that the color-mix function
will treat it as having the same hue as
green
(effectively
hsl(120deg 0% 100%)
),
and then interpolate based on those components.
The result will be a color that truly looks like a blend of green and white,
rather than perhaps looking reddish
(if
white
s hue was defaulted to
0deg
).
Explicitly specifying missing components can be useful
to achieve an effect where you only
want
to interpolate certain components of a color.
For example, to animate a color to "grayscale", no matter what the color is,
one can interpolate it with
oklch(none 0 none)
This will take the hue and lightness from the starting color,
but animate its chroma down to 0,
rendering it into an equal-lightness gray
with a steady hue across the whole animation.
Doing this manually would require
matching the hue and lightness of the starting color explicitly.
4.4.1.
“Powerless” Color Components
Individual color syntaxes can specify that,
in some cases,
a given component of their syntax becomes a
powerless color component
This indicates that the value of the component doesn’t affect the rendered color;
any value you give it will result in the same color displayed in the screen.
For example, in
hsl()
, the hue component is
powerless
when the saturation component is
0%
0%
saturation indicates a grayscale color,
which has no hue at all,
so
0deg
and
180deg
, or any other angle,
will give the exact same result.
If a
powerless component
is manually specified,
it acts as normal;
the fact that it’s
powerless
has no effect.
However, if a color is automatically produced by color space conversion,
then any
powerless components
in the result must instead be set to
missing
instead of whatever value was produced by the conversion process.
When performing color space conversion to a
cylindrical polar color
space,
user agents
shall
treat a hue component as
powerless
if the chroma (or other measure of colorfulness, such as saturation in
hsl
is less than the epsilon (ε) specified for that color space.
For example, a gray color converted into
oklch()
may,
due to numerical errors,
have an
extremely small
chroma rather than precisely
0%
as a result, the hue component is
powerless
4.5.
Parsing a

Value
Tests
This section provides a definition referenced elsewhere, it does not need tests.
To
parse a CSS

value
given a
string
input
and an optional context
element
element
Parse
input
as a

If the result is failure,
return failure;
otherwise, let
color
be the result.
Let
used color
be the result of
resolving
color
to a
used color
If the value of other properties
on the element a

is on
is required to do the resolution
(such as resolving a
currentcolor
or
system color
),
use
element
if it was passed,
or the
initial values
of the properties if not.
Return
used color
Note:
This algorithm is not intented
to parse a CSS

value
specified in a CSS stylesheet
or with a CSSOM interface,
but in other places
like HTML attributes or Canvas interfaces.
5.
sRGB Colors
CSS colors in the
sRGB
color space
are represented by a triplet of values—​red, green, and blue—​identifying a point in the sRGB color space
[SRGB]
This is an internationally-recognized, device-independent color space,
and so is useful for specifying colors that will be displayed on a computer screen,
but is also useful for specifying colors on other types of devices, like printers.
CSS also allows the use of non-sRGB
color space
s,
as described in
§ 10 Predefined Color Spaces
CSS provides several methods of directly specifying an sRGB color:
hex colors
rgb()
rgba()
color functions
hsl()
hsla()
color functions
hwb()
color function
named colors
and the
transparent
keyword.
5.1.
The RGB functions:
rgb()
and
rgba()
The
rgb()
and
rgba()
functions define an sRGB color
by specifying the r, g and b (red, green, and blue) components directly.
Their syntax is:
rgb
()


rgba
()



rgb


rgb



rgba


rgba



rgb


none

none

rgba


none

none
Percentages
Allowed for r, g and b
Percent reference range
For r, g and b: 0% = 0.0, 100% = 255.0
For alpha: 0% = 0.0, 100% = 1.0
Tests
rgb-001.html
(live test)
(source)
rgb-002.html
(live test)
(source)
rgb-003.html
(live test)
(source)
rgb-004.html
(live test)
(source)
rgb-005.html
(live test)
(source)
rgb-006.html
(live test)
(source)
rgb-007.html
(live test)
(source)
rgb-008.html
(live test)
(source)
out-of-gamut-legacy-rgb.html
(live test)
(source)
color-valid.html
(live test)
(source)
color-computed-rgb.html
(live test)
(source)
color-invalid-rgb.html
(live test)
(source)
color-valid-rgb.html
(live test)
(source)
The first three arguments specify the r, g and b (red, green, and blue)
components of the color, respectively.
0%
represents the minimum value for that color component in the sRGB gamut,
and
100%
represents the maximum value.
The percentage reference range of the color components comes from the historical fact that
many graphics engines stored the color components internally as a single byte,
which can hold integers between 0 and 255.
Implementations should honor the precision of the component as authored or calculated wherever possible.
If this is not possible, the component should be
rounded towards +∞
The final argument, the

, specifies the alpha of the color.
If omitted, it defaults to
100%
Tests
background-color-rgb-001.html
(live test)
(source)
background-color-rgb-002.html
(live test)
(source)
background-color-rgb-003.html
(live test)
(source)
color-valid.html
(live test)
(source)
Values outside these ranges are not invalid,
but are clamped to the ranges defined here at parsed-value time.
For historical reasons,
rgb()
and
rgba()
also support a
legacy color syntax
Tests
rgba-001.html
(live test)
(source)
rgba-002.html
(live test)
(source)
rgba-003.html
(live test)
(source)
rgba-004.html
(live test)
(source)
rgba-005.html
(live test)
(source)
rgba-006.html
(live test)
(source)
rgba-007.html
(live test)
(source)
rgba-008.html
(live test)
(source)
color-valid.html
(live test)
(source)
5.2.
The RGB Hexadecimal Notations:
#RRGGBB
The CSS
hex color notation
allows an sRGB color to be specified by giving the components as hexadecimal numbers,
which is similar to how colors are often written directly in computer code.
It’s also shorter than writing the same color out in
rgb()
notation.
The syntax of a

is a

token whose value consists of 3, 4, 6, or 8 hexadecimal digits.
In other words, a hex color is written as a hash character, "#",
followed by some number of digits 0-9 or letters a-f
(the case of the letters doesn’t matter -
#00ff00
is identical to
#00FF00
).
The number of hex digits given determines how to decode the hex notation into an RGB color:
6 digits
The first pair of digits, interpreted as a hexadecimal number,
specifies the red component of the color,
where
00
represents the minimum value
and
ff
(255 in decimal) represents the maximum.
The next pair of digits, interpreted in the same way,
specifies the green component,
and the last pair specifies the blue.
The alpha component of the color is fully opaque.
In other words,
#00ff00
represents the same color as
rgb(0 255 0)
(a lime green).
8 digits
The first 6 digits are interpreted identically to the 6-digit notation.
The last pair of digits, interpreted as a hexadecimal number,
specifies the alpha component of the color,
where
00
represents a fully transparent color
and
ff
represent a fully opaque color.
In other words,
#0000ffcc
represents the same color as
rgb(0 0 100% / 80%)
(a slightly-transparent blue).
3 digits
This is a shorter variant of the 6-digit notation.
The first digit, interpreted as a hexadecimal number,
specifies the red component of the color,
where
represents the minimum value
and
represents the maximum.
The next two digits represent the green and blue components, respectively,
in the same way.
The alpha component of the color is fully opaque.
This syntax is often explained by saying that it’s identical to a 6-digit notation obtained by "duplicating" all of the digits.
For example, the notation
#123
specifies the same color as the notation
#112233
This method of specifying a color has lower "resolution" than the 6-digit notation;
there are only 4096 possible colors expressible in the 3-digit hex syntax,
as opposed to approximately 17 million in 6-digit hex syntax.
4 digits
This is a shorter variant of the 8-digit notation,
"expanded" in the same way as the 3-digit notation is.
The first digit, interpreted as a hexadecimal number,
specifies the red component of the color,
where
represents the minimum value
and
represents the maximum.
The next three digits represent the green, blue, and alpha components, respectively.
Tests
hex-001.html
(live test)
(source)
hex-002.html
(live test)
(source)
hex-003.html
(live test)
(source)
hex-004.html
(live test)
(source)
border-bottom-color.xht
(live test)
(source)
border-left-color.xht
(live test)
(source)
border-right-color.xht
(live test)
(source)
border-top-color.xht
(live test)
(source)
color-valid.html
(live test)
(source)
color-computed-hex-color.html
(live test)
(source)
color-invalid-hex-color.html
(live test)
(source)
6.
Color Keywords
In addition to the various numeric syntaxes for

s,
CSS defines several sets of color keywords that can be used instead—​each with their own advantages or use cases.
6.1.
Named Colors
CSS defines a large set of
named colors
so that common colors can be written and read more easily.

is written as an

accepted anywhere a

is.
As usual for CSS-defined

s,
all of these keywords are
ASCII case-insensitive
The names resolve to colors in sRGB.
16 of CSS’s named colors come from the VGA palette originally, and were then adopted into HTML:
aqua, black, blue, fuchsia, gray, green, lime, maroon, navy, olive, purple, red, silver, teal, white, and yellow.
Most of the rest
come from one version of the X11 color system,
used in Unix-derived systems to specify colors for the console,
and were then adopted into SVG.
Note:
these color names are standardized here,
not because they are good
but because their use and implementation has been widespread for decades
and the standard needs to reflect reality.
Indeed, it is often hard to imagine what each name will look like (hence the list below);
the names are not evenly distributed throughout the sRGB color volume,
the names are not even internally consistent
darkgray
is lighter than
gray
, while
lightpink
is darker than
pink
),
and some names
(such as
indianred
which was originally named after a red pigment from India),
have been found to be offensive.
Thus, their use is
not encouraged
(Two special color values,
transparent
and
currentcolor
are specially defined in their own sections.)
The following table defines all of the opaque named colors,
by giving equivalent numeric specifications in the other color syntaxes.
Named
Numeric
Color name
Hex rgb
Decimal
aliceblue
#f0f8ff
240 248 255
antiquewhite
#faebd7
250 235 215
aqua
#00ffff
0 255 255
aquamarine
#7fffd4
127 255 212
azure
#f0ffff
240 255 255
beige
#f5f5dc
245 245 220
bisque
#ffe4c4
255 228 196
black
#000000
0 0 0
blanchedalmond
#ffebcd
255 235 205
blue
#0000ff
0 0 255
blueviolet
#8a2be2
138 43 226
brown
#a52a2a
165 42 42
burlywood
#deb887
222 184 135
cadetblue
#5f9ea0
95 158 160
chartreuse
#7fff00
127 255 0
chocolate
#d2691e
210 105 30
coral
#ff7f50
255 127 80
cornflowerblue
#6495ed
100 149 237
cornsilk
#fff8dc
255 248 220
crimson
#dc143c
220 20 60
cyan
#00ffff
0 255 255
darkblue
#00008b
0 0 139
darkcyan
#008b8b
0 139 139
darkgoldenrod
#b8860b
184 134 11
darkgray
#a9a9a9
169 169 169
darkgreen
#006400
0 100 0
darkgrey
#a9a9a9
169 169 169
darkkhaki
#bdb76b
189 183 107
darkmagenta
#8b008b
139 0 139
darkolivegreen
#556b2f
85 107 47
darkorange
#ff8c00
255 140 0
darkorchid
#9932cc
153 50 204
darkred
#8b0000
139 0 0
darksalmon
#e9967a
233 150 122
darkseagreen
#8fbc8f
143 188 143
darkslateblue
#483d8b
72 61 139
darkslategray
#2f4f4f
47 79 79
darkslategrey
#2f4f4f
47 79 79
darkturquoise
#00ced1
0 206 209
darkviolet
#9400d3
148 0 211
deeppink
#ff1493
255 20 147
deepskyblue
#00bfff
0 191 255
dimgray
#696969
105 105 105
dimgrey
#696969
105 105 105
dodgerblue
#1e90ff
30 144 255
firebrick
#b22222
178 34 34
floralwhite
#fffaf0
255 250 240
forestgreen
#228b22
34 139 34
fuchsia
#ff00ff
255 0 255
gainsboro
#dcdcdc
220 220 220
ghostwhite
#f8f8ff
248 248 255
gold
#ffd700
255 215 0
goldenrod
#daa520
218 165 32
gray
#808080
128 128 128
green
#008000
0 128 0
greenyellow
#adff2f
173 255 47
grey
#808080
128 128 128
honeydew
#f0fff0
240 255 240
hotpink
#ff69b4
255 105 180
indianred
#cd5c5c
205 92 92
indigo
#4b0082
75 0 130
ivory
#fffff0
255 255 240
khaki
#f0e68c
240 230 140
lavender
#e6e6fa
230 230 250
lavenderblush
#fff0f5
255 240 245
lawngreen
#7cfc00
124 252 0
lemonchiffon
#fffacd
255 250 205
lightblue
#add8e6
173 216 230
lightcoral
#f08080
240 128 128
lightcyan
#e0ffff
224 255 255
lightgoldenrodyellow
#fafad2
250 250 210
lightgray
#d3d3d3
211 211 211
lightgreen
#90ee90
144 238 144
lightgrey
#d3d3d3
211 211 211
lightpink
#ffb6c1
255 182 193
lightsalmon
#ffa07a
255 160 122
lightseagreen
#20b2aa
32 178 170
lightskyblue
#87cefa
135 206 250
lightslategray
#778899
119 136 153
lightslategrey
#778899
119 136 153
lightsteelblue
#b0c4de
176 196 222
lightyellow
#ffffe0
255 255 224
lime
#00ff00
0 255 0
limegreen
#32cd32
50 205 50
linen
#faf0e6
250 240 230
magenta
#ff00ff
255 0 255
maroon
#800000
128 0 0
mediumaquamarine
#66cdaa
102 205 170
mediumblue
#0000cd
0 0 205
mediumorchid
#ba55d3
186 85 211
mediumpurple
#9370db
147 112 219
mediumseagreen
#3cb371
60 179 113
mediumslateblue
#7b68ee
123 104 238
mediumspringgreen
#00fa9a
0 250 154
mediumturquoise
#48d1cc
72 209 204
mediumvioletred
#c71585
199 21 133
midnightblue
#191970
25 25 112
mintcream
#f5fffa
245 255 250
mistyrose
#ffe4e1
255 228 225
moccasin
#ffe4b5
255 228 181
navajowhite
#ffdead
255 222 173
navy
#000080
0 0 128
oldlace
#fdf5e6
253 245 230
olive
#808000
128 128 0
olivedrab
#6b8e23
107 142 35
orange
#ffa500
255 165 0
orangered
#ff4500
255 69 0
orchid
#da70d6
218 112 214
palegoldenrod
#eee8aa
238 232 170
palegreen
#98fb98
152 251 152
paleturquoise
#afeeee
175 238 238
palevioletred
#db7093
219 112 147
papayawhip
#ffefd5
255 239 213
peachpuff
#ffdab9
255 218 185
peru
#cd853f
205 133 63
pink
#ffc0cb
255 192 203
plum
#dda0dd
221 160 221
powderblue
#b0e0e6
176 224 230
purple
#800080
128 0 128
rebeccapurple
#663399
102 51 153
red
#ff0000
255 0 0
rosybrown
#bc8f8f
188 143 143
royalblue
#4169e1
65 105 225
saddlebrown
#8b4513
139 69 19
salmon
#fa8072
250 128 114
sandybrown
#f4a460
244 164 96
seagreen
#2e8b57
46 139 87
seashell
#fff5ee
255 245 238
sienna
#a0522d
160 82 45
silver
#c0c0c0
192 192 192
skyblue
#87ceeb
135 206 235
slateblue
#6a5acd
106 90 205
slategray
#708090
112 128 144
slategrey
#708090
112 128 144
snow
#fffafa
255 250 250
springgreen
#00ff7f
0 255 127
steelblue
#4682b4
70 130 180
tan
#d2b48c
210 180 140
teal
#008080
0 128 128
thistle
#d8bfd8
216 191 216
tomato
#ff6347
255 99 71
turquoise
#40e0d0
64 224 208
violet
#ee82ee
238 130 238
wheat
#f5deb3
245 222 179
white
#ffffff
255 255 255
whitesmoke
#f5f5f5
245 245 245
yellow
#ffff00
255 255 0
yellowgreen
#9acd32
154 205 50
Note:
this list of colors and their definitions is a superset of the list of
named colors defined by SVG 1.1
For historical reasons, this is also referred to as the X11 color set.
Note:
The history of the X11 color system is interesting,
and was excellently summarized by
Alex Sexton in their talk “Peachpuffs and Lemonchiffons”
Tests
named-001.html
(live test)
(source)
color-valid.html
(live test)
(source)
color-computed-named-color.html
(live test)
(source)
color-invalid-named-color.html
(live test)
(source)
6.2.
System Colors
In general, the

keywords
reflect
default
color choices made by the user, the browser, or the OS.
They are typically used in the browser default stylesheet, for this reason.
To maintain legibility,
the

keywords also respond to
the
used color scheme
For example, traditional
blue link text is legible on a
white background
(WCAG contrast 8.59:1, AAA pass)
but would not be legible on a
black background
(WCAG contrast 2.44:1, AA fail).
Instead, a lighter blue such as
#81D9FE would be used in dark mode
(WCAG contrast 13.28:1, AAA pass).
Legible link text
Illegible link text
Legible link text
However, in
forced colors mode
most colors on the page are forced into a restricted, user-chosen palette,
see
CSS Color Adjustment 1
§ 5.2 Forced Colors Mode Color Palettes
The

keywords
expose these user-chosen colors
so that the rest of the page can integrate with this restricted palette.
When the
forced-colors
media feature
is
active
authors
should
use the

keywords as color values
in properties other than those listed in
CSS Color Adjustment 1
§ 3.1 Properties Affected by Forced Colors Mode
to ensure legibility and consistency across the page
and avoid an uncoordinated mishmash of user-forced and page-chosen colors.
Tests
system-color-consistency.html
(live test)
(source)
system-color-support.html
(live test)
(source)
color-valid-system-color.html
(live test)
(source)
When the values of

keywords come from the browser,
(as opposed to being OS defaults or user choices) the browser should
ensure that
matching
foreground/background pairs
have a minimum
of WCAG AA contrast.
However, user preferences (for higher or lower contrast),
whether set as a browser preference, a user stylesheet,
or by altering the OS defaults,
must take precedence over this requirement.
Authors
may
also use these keywords at any time,
but
should
be careful to use the colors
in
matching background-foreground pairs
to ensure appropriate contrast,
as any particular contrast relationship across non-matching pairs
(e.g.
Canvas
and
ButtonText
is not guaranteed.
The

keywords are defined as follows:
AccentColor
Background of accented user interface controls.
AccentColorText
Text of accented user interface controls.
ActiveText
Text in active links. For light backgrounds, traditionally red.
ButtonBorder
The base border color for push buttons.
ButtonFace
The face background color for push buttons.
ButtonText
Text on push buttons.
Canvas
Background of application content or documents.
CanvasText
Text in application content or documents.
Field
Background of input fields.
FieldText
Text in input fields.
GrayText
Disabled text.
(Often, but not necessarily, gray.)
Highlight
Background of selected text, for example from ::selection.
HighlightText
Text of selected text.
LinkText
Text in non-active, non-visited links. For light backgrounds, traditionally blue.
Mark
Background of text that has been specially marked
(such as by the HTML
mark
element).
MarkText
Text that has been specially marked
(such as by the HTML
mark
element).
SelectedItem
Background of selected items, for example a selected checkbox.
SelectedItemText
Text of selected items.
VisitedText
Text in visited links. For light backgrounds, traditionally purple.
Tests
color-valid.html
(live test)
(source)
relative-currentcolor-visited-getcomputedstyle.html
(live test)
(source)
system-color-compute.html
(live test)
(source)
system-color-hightlights-vs-getSelection-001.html
(live test)
(source)
system-color-hightlights-vs-getSelection-002.html
(live test)
(source)
Note:
As with all other
keywords
these names are
ASCII case-insensitive
They are shown here with mixed capitalization for legibility.
For systems that do not have a particular system UI concept,
the specified value should be mapped to
the most closely related system color value that exists.
The following
system color pairings
are expected to form legible background-foreground colors:
Canvas
background with
CanvasText
LinkText
VisitedText
ActiveText
foreground.
Canvas
background with a
ButtonBorder
border and adjacent color
Canvas
ButtonFace
background with
ButtonText
foreground.
Field
background with
FieldText
foreground.
Mark
background with
MarkText
foreground
ButtonFace
or
Field
background with a
ButtonBorder
border and adjacent color
Canvas
Highlight
background with
HighlightText
foreground.
SelectedItem
background with
SelectedItemText
foreground.
AccentColor
background with
AccentColorText
foreground.
Additionally,
GrayText
is expected to be readable,
though possibly at a lower contrast rating,
over any of the backgrounds.
To maintain consistency with widget
accent color
styling,
AccentColor
takes its value from
accent-color
unless
Forced Colors Mode
is enabled.
AccentColorText
takes its value from
the contrasting foreground color to
AccentColor
as is described for widget
accent color
styling.
For example, the system color combinations in the browser you are currently using:
Canvas with CanvasText:
CanvasText
Canvas with LinkText:
LinkText
Canvas with VisitedText:
VisitedText
Canvas with ActiveText:
ActiveText
Canvas with GrayText:
GrayText
Canvas with ButtonBorder and adjacent Canvas:
CanvasText
Adjacent
ButtonFace with ButtonText:
ButtonText
ButtonFace with ButtonText and ButtonBorder:
ButtonText
ButtonFace with GrayText:
GrayText
Field with FieldText:
FieldText
Field with GrayText:
GrayText
Mark with MarkText:
MarkText
Mark with GrayText:
GrayText
Highlight with HighlightText:
HighlightText
Highlight with GrayText:
GrayText
SelectedItem with SelectedItemText:
SelectedItemText
AccentColor with AccentColorText:
AccentColorText
AccentColor with GrayText:
GrayText
Earlier versions of CSS defined additional

s,
which have since been deprecated.
These are documented in
Appendix A: Deprecated CSS System Colors
Note:
The

s incur some privacy and security risk, as detailed in
§ 22 Privacy Considerations
and
§ 21 Security Considerations
User agents may,
to mitigate privacy and security risks such as fingerprinting,
elect to return fixed values for the used value of system colors
which do not reflect customisation or theming choices
made by the user.
6.3.
The
transparent
keyword
The keyword
transparent
specifies a
transparent black
It is a type of

Tests
color-computed.html
(live test)
(source)
color-valid.html
(live test)
(source)
t423-transparent-1-a.xht
(live test)
(source)
t423-transparent-2-a.xht
(live test)
(source)
6.4.
The
currentcolor
keyword
The keyword
currentcolor
represents value of the
color
property on the same element.
Unlike

s, it is
not
restricted to sRGB;
the value can be any

Its
used values
is determined by
resolving color values
Tests
border-color-currentcolor.html
(live test)
(source)
color-mix-currentcolor-nested-for-color-property.html
(live test)
(source)
currentcolor-001.html
(live test)
(source)
currentcolor-002.html
(live test)
(source)
currentcolor-003.html
(live test)
(source)
currentcolor-004.html
(live test)
(source)
currentcolor-visited-fallback.html
(live test)
(source)
color-valid.html
(live test)
(source)
Here’s a simple example showing how to use the
currentcolor
keyword:
.foo
color
red
background-color
currentcolor
This is equivalent to writing:
.foo
color
red
background-color
red
For example, the
text-emphasis-color
property
[CSS3-TEXT-DECOR]
whose initial value is
currentcolor
by default matches the text color
even as the
color
property changes across elements.
><
em
Some
strong
really
strong
emphasized text.
em
style
color
black
em
text-emphasis
dot
strong
color
red
style
In the above example, the emphasis marks are black over the text "Some" and "emphasized text",
but red over the text "really".
Note:
Multi-word keywords in CSS usually separate their component words with hyphens.
currentcolor
doesn’t, because (deep breath)
it was originally introduced in SVG
as a property value, "current-color" with the usual CSS spelling.
It (along with all other properties and their values)
then became presentation attributes and attribute values,
as well as properties,
to make generation with XSLT easier.
Then all of the presentation attributes were changed
from hyphenated to camelCase, because the DOM
had an issue with hyphen meaning "minus".
But then, they didn’t follow CSS conventions
anymore so all the properties and property values
that were
already
part of CSS were changed back to hyphenated!
currentcolor
was not a part of CSS at that time,
so remained camelCased.
Only later did CSS pick it up,
at which point the capitalization stopped mattering,
as CSS keywords are
ASCII case-insensitive
7.
HSL Colors:
hsl()
and
hsla()
functions
The RGB system for specifying colors,
while convenient for machines and graphic libraries,
is often regarded as very difficult for humans to gain an intuitive grasp on.
It’s not easy to tell, for example,
how to alter an RGB color to produce a lighter variant of the same hue.
There are several other color schemes possible.
One such is the HSL
[HSL]
color scheme,
which is more intuitive to use,
but still maps easily back to RGB colors.
HSL
colors are specified
as a triplet of hue, saturation, and lightness.
The syntax of the
hsl()
and
hsla()
functions is:
hsl
()


hsla
()



hsl

none


none


none

none

hsla

none


none


none

none

hsl





hsla




Percentages
Allowed for S and L
Percent reference range
for S and L: 0% = 0.0, 100% = 100.0
Powerless hue ε
S <= 0.001
Tests
hsl-001.html
(live test)
(source)
hsl-002.html
(live test)
(source)
hsl-003.html
(live test)
(source)
hsl-004.html
(live test)
(source)
hsl-005.html
(live test)
(source)
hsl-006.html
(live test)
(source)
hsl-007.html
(live test)
(source)
hsl-008.html
(live test)
(source)
hsl-clamp-negative-saturation.html
(live test)
(source)
background-color-hsl-001.html
(live test)
(source)
background-color-hsl-002.html
(live test)
(source)
background-color-hsl-003.html
(live test)
(source)
background-color-hsl-004.html
(live test)
(source)
color-computed-hsl.html
(live test)
(source)
color-invalid-hsl.html
(live test)
(source)
color-valid-hsl.html
(live test)
(source)
The first argument specifies the hue angle.
In HSL (and HWB) the angle
0deg
represents sRGB primary red
(as does
360deg
720deg
, etc.),
and the rest of the hues are spread around the circle,
so
120deg
represents sRGB primary green,
240deg
represents sRGB primary blue, etc.
The next two arguments are the saturation and lightness, respectively.
For saturation,
100%
or
100
is a fully-saturated, bright color,
and
0%
or
is a fully-unsaturated gray.
For lightness,
50%
or
50
represents the "normal" color,
while
100%
or
100
is white and
0%
or
is black.
For historical reasons,
if the saturation is less than
0%
it is clamped to
0%
at parsed-value time,
before being converted to an sRGB color.
Tests
color-valid-hsl.html
(live test)
(source)
The final argument specifies the alpha component of the color.
It’s interpreted identically to the fourth argument of the
rgb()
function.
If omitted, it defaults to
100%
HSL colors resolve to sRGB.
If the saturation of an HSL color is
0%
or
then the hue component is
powerless
For example, an ordinary red,
the same color you would see from the keyword
red
or the hex notation
#f00
is represented in HSL as
hsl(0deg 100% 50%)
An advantage of HSL over RGB is that it is more intuitive:
people can guess at the colors they want,
and then tweak.
For example, the following colors can all be generated off of the basic "green" hue,
just by varying the other two arguments:
hsl
120
deg
100
50
lime green
hsl
120
deg
100
25
dark green
hsl
120
deg
100
75
light green
hsl
120
deg
75
85
pastel green
A disadvantage of HSL over OkLCh
is that hue manipulation changes the visual lightness,
and that hues are not evenly spaced apart.
It is thus easier in HSL to create sets of matching colors
(by keeping the hue the same and varying the saturation and lightness),
compared to manipulating the sRGB component values;
however, because the lightness is simply the mean of the gamma-corrected
red, green and blue components
it does not correspond to the visual perception of lightness
across hues.
For example,
blue
is represented in HSL as
hsl(240deg 100% 50%)
while
yellow
is
hsl(60deg 100% 50%)
Both have an HSL Lightness of 50%,
but clearly the yellow looks much lighter than the blue.
In OkLCh, sRGB blue is
oklch(0.452 0.313 264.1)
while
sRGB yellow is
oklch(0.968 0.211 109.8)
The OkLCh Lightnesses of 0.452 and 0.968 clearly reflect
the visual lightnesses of the two colors.
The hue angle in HSL is not perceptually uniform;
colors appear bunched up in some areas
and widely spaced in others.
For example, the pair of hues
hsl(220deg 100% 50%)
and
hsl(250deg 100% 50%)
have an HSL hue difference of 250-220 =
30
deg and look fairly similar,
while another pair of colors
hsl(50deg 100% 50%)
and
hsl(80deg 100% 50%)
which
also
have a hue difference of 80-50 =
30
deg, look very different.
In OkLCh, the same pair of colors
oklch(0.533 0.26 262.6)
and
oklch(0.462 0.306 268.9)
have a hue difference of 268.9 - 262.6 =
6.3
deg
while the second pair
oklch(0.882 0.181 94.24)
and
oklch(0.91 0.245 129.9)
have a hue difference of 129.9 - 94.24 =
35.66
deg,
correctly reflecting the visual separation of hues.
For historical reasons,
hsl()
and
hsla()
also support a
legacy color syntax
Tests
hsla-001.html
(live test)
(source)
hsla-002.html
(live test)
(source)
hsla-003.html
(live test)
(source)
hsla-004.html
(live test)
(source)
hsla-005.html
(live test)
(source)
hsla-006.html
(live test)
(source)
hsla-007.html
(live test)
(source)
hsla-008.html
(live test)
(source)
hsla-clamp-negative-saturation.html
(live test)
(source)
color-valid.html
(live test)
(source)
7.1.
Converting HSL Colors to sRGB
Converting an HSL color to sRGB is straightforward mathematically.
Here’s a sample implementation of the conversion algorithm in JavaScript.
It returns an array of three numbers
representing the red, green, and blue components of the colors,
which for colors in the sRGB gamut will be in the range [0, 1].
This code assumes that
parse-time
clamping
of negative saturation has already been applied.
/**
* @param {number} hue - Hue as degrees 0..360
* @param {number} sat - Saturation in reference range [0,100]
* @param {number} light - Lightness in reference range [0,100]
* @return {number[]} Array of sRGB components; in-gamut colors in range [0..1]
*/
function
hslToRgb
hue
sat
light
sat
/=
100
light
/=
100
function
let
hue
30
12
let
sat
Math
min
light
light
);
return
light
Math
max
Math
min
));
return
),
),
)];
7.2.
Converting sRGB Colors to HSL
Conversion in the reverse direction proceeds similarly.
Special care is taken to deal with
intermediate negative values of saturation,
which can be produced by colors far outside the sRGB gamut.
/**
* @param {number} red - Red component 0..1
* @param {number} green - Green component 0..1
* @param {number} blue - Blue component 0..1
* @return {number[]} Array of HSL values: Hue as degrees 0..360, Saturation and Lightness in reference range [0,100]
*/
function
rgbToHsl
red
green
blue
let
max
Math
max
red
green
blue
);
let
min
Math
min
red
green
blue
);
let
hue
sat
light
NaN
min
max
];
let
max
min
let
epsilon
100000
// max Sat is 1, in this code
if
!==
sat
light
===
||
light
===
max
light
Math
min
light
light
);
switch
max
case
red
hue
green
blue
green
blue
);
break
case
green
hue
blue
red
break
case
blue
hue
red
green
hue
hue
60
// Very out of gamut colors can produce negative saturation
// If so, just rotate the hue by 180 and use a positive saturation
// see https://github.com/w3c/csswg-drafts/issues/9222
if
sat
hue
+=
180
sat
Math
abs
sat
);
if
hue
>=
360
hue
-=
360
if
sat
<=
epsilon
hue
NaN
return
hue
sat
100
light
100
];
7.3.
Examples of HSL Colors
This section is not normative.
Tests
This section is not normative, it does not need tests.
The tables below illustrate a wide range of possible HSL colors.
Each table represents one hue,
selected at 30° intervals,
to illustrate the common "core" hues:
red,
yellow,
green,
cyan,
blue,
magenta,
and the six intermediary colors between these.
In each table, the X axis represents the saturation
while the Y axis represents the lightness.
0° Reds
100%
80%
60%
40%
20%
0%
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
30° Reds-Yellows (=Oranges)
100%
80%
60%
40%
20%
0%
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
60° Yellows
100%
80%
60%
40%
20%
0%
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
90° Yellow-Greens
100%
80%
60%
40%
20%
0%
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
120° Greens
100%
80%
60%
40%
20%
0%
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
150° Green-Cyans
100%
80%
60%
40%
20%
0%
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
180° Cyans
100%
80%
60%
40%
20%
0%
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
210° Cyan-Blues
100%
80%
60%
40%
20%
0%
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
240° blues
100%
80%
60%
40%
20%
0%
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
270° Blue-Magentas
100%
80%
60%
40%
20%
0%
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
300° Magentas
100%
80%
60%
40%
20%
0%
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
330° Magenta-Reds
100%
80%
60%
40%
20%
0%
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
8.
HWB Colors:
hwb()
function
HWB
(short for Hue-Whiteness-Blackness)
[HWB]
is another method of specifying sRGB colors,
similar to
HSL
', but often even easier for humans to work with.
It describes colors with a starting hue,
then a degree of whiteness and blackness to mix into that base hue.
Many color-pickers are based on the HWB color system,
due to its intuitiveness.
HWB colors resolve to sRGB.
This is a screenshot of Chrome’s color picker,
shown when a user activates an
input
type
"color"
The outer wheel is used to select the hue,
then the relative amounts of white and black are selected by clicking on the inner triangle.
The syntax of the
hwb()
function is:
hwb
()
hwb

none


none


none

none
Percentages
Allowed for W and B
Percent reference range
for W and B: 0% = 0.0, 100% = 100.0
Powerless hue ε
W + B >= 99.999
The first argument specifies the hue,
and is defined identically to
hsl()
this means it
suffers the same disadvantages
such as hue uniformity.
The second argument specifies the amount of white to mix in,
as a percentage from
0%
(no whiteness) to
100%
(full whiteness).
Similarly, the third argument specifies the amount of black to mix in,
also from
0%
(no blackness) to
100%
(full blackness).
For example,
hwb(150 20% 10%)
is the same color as
hsl(150 77.78% 55%)
and
rgb(20% 90% 55%).
Values outside of these ranges
are not invalid;
hue angles outside the range [0,360) will be normalized to that range
and values of white and black which sum to 100% or greater will
produce achromatic colors as described below.
The resulting color can be thought of conceptually as a mixture of paint in the chosen hue,
white paint, and black paint,
with the relative amounts of each determined by the percentages.
If the sum white+black is greater than or equal to
100%
it defines an achromatic color,
i.e. a shade of gray;
when converted to sRGB the R, G and B values are identical
and have the value white / (white + black).
For example, in the color
hwb(45 40% 80%)
white and black adds to 120, so this is an achromatic color
whose R, G and B components are 40 / 40 + 80 = 0.33
rgb(33.33% 33.33% 33.33%).
Achromatic HWB colors no longer contain any hint of the chosen hue.
In this case, the hue component is
powerless
The fourth argument specifies the alpha component of the color.
It’s interpreted identically to the fourth argument of the
rgb()
function.
If omitted, it defaults to
100%
There is no Web compatibility issue
with
hwb
, which is new in this level of the specification, and so
hwb()
does
not
support a
legacy color syntax
that separates all of its arguments with commas.
Using commas inside
hwb()
is an error.
Tests
hwb-001.html
(live test)
(source)
hwb-002.html
(live test)
(source)
hwb-003.html
(live test)
(source)
hwb-004.html
(live test)
(source)
hwb-005.html
(live test)
(source)
color-valid.html
(live test)
(source)
color-computed-hwb.html
(live test)
(source)
color-invalid-hwb.html
(live test)
(source)
color-valid-hwb.html
(live test)
(source)
8.1.
Converting HWB Colors to sRGB
Converting an HWB color to sRGB is straightforward,
and related to how one converts HSL to RGB.
The following Javascript implementation of the algorithm
first normalizes the white and black components,
so their sum is no larger than 100%.
/**
* @param {number} hue - Hue as degrees 0..360
* @param {number} white - Whiteness in reference range [0,100]
* @param {number} black - Blackness in reference range [0,100]
* @return {number[]} Array of RGB components 0..1
*/
function
hwbToRgb
hue
white
black
white
/=
100
black
/=
100
if
white
black
>=
let
gray
white
white
black
);
return
gray
gray
gray
];
let
rgb
hslToRgb
hue
100
50
);
for
let
++
rgb
*=
white
black
);
rgb
+=
white
return
rgb
8.2.
Converting sRGB Colors to HWB
Conversion in the reverse direction proceeds similarly.
/**
* @param {number} red - Red component 0..1
* @param {number} green - Green component 0..1
* @param {number} blue - Blue component 0..1
* @return {number} Hue as degrees 0..360
*/
function
rgbToHue
red
green
blue
// Similar to rgbToHsl, except that saturation and lightness are not calculated, and
// potential negative saturation is ignored.
let
max
Math
max
red
green
blue
);
let
min
Math
min
red
green
blue
);
let
hue
NaN
let
max
min
if
!==
switch
max
case
red
hue
green
blue
green
blue
);
break
case
green
hue
blue
red
break
case
blue
hue
red
green
hue
*=
60
if
hue
>=
360
hue
-=
360
return
hue
/**
* @param {number} red - Red component 0..1
* @param {number} green - Green component 0..1
* @param {number} blue - Blue component 0..1
* @return {number[]} Array of HWB values: Hue as degrees 0..360, Whiteness and Blackness in reference range [0,100]
*/
function
rgbToHwb
red
green
blue
let
epsilon
100000
// account for multiply by 100
var
hue
rgbToHue
red
green
blue
);
var
white
Math
min
red
green
blue
);
var
black
Math
max
red
green
blue
);
if
white
black
>=
epsilon
hue
NaN
return
([
hue
white
100
black
100
]);
8.3.
Examples of HWB Colors
This section is not normative.
Tests
This section is not normative, it does not need tests.
0° Reds
0%
20%
40%
60%
80%
100%
0%
20%
40%
60%
80%
100%
30° Red-Yellows (Oranges)
0%
20%
40%
60%
80%
100%
0%
20%
40%
60%
80%
100%
60° Yellows
0%
20%
40%
60%
80%
100%
0%
20%
40%
60%
80%
100%
90° Yellow-Greens
0%
20%
40%
60%
80%
100%
0%
20%
40%
60%
80%
100%
120° Greens
0%
20%
40%
60%
80%
100%
0%
20%
40%
60%
80%
100%
150° Green-Cyans
0%
20%
40%
60%
80%
100%
0%
20%
40%
60%
80%
100%
180° Cyans
0%
20%
40%
60%
80%
100%
0%
20%
40%
60%
80%
100%
210° Cyan-Blues
0%
20%
40%
60%
80%
100%
0%
20%
40%
60%
80%
100%
240° Blues
0%
20%
40%
60%
80%
100%
0%
20%
40%
60%
80%
100%
270° Blue-Magentas
0%
20%
40%
60%
80%
100%
0%
20%
40%
60%
80%
100%
300° Magentas
0%
20%
40%
60%
80%
100%
0%
20%
40%
60%
80%
100%
330° Magenta-Reds
0%
20%
40%
60%
80%
100%
0%
20%
40%
60%
80%
100%
9.
Device-independent Colors: CIE Lab and LCH, Oklab and OkLCh
9.1.
CIE Lab and LCH
This section is not normative.
Tests
This section is not normative, it does not need tests.
Physical measurements of a color are typically expressed in the CIE L*a*b*
[CIELAB]
color space,
created in 1976 by the
CIE
and commonly referred to simply as Lab.
Color conversions from one device to another may also use Lab as an intermediate step.
Derived from human vision experiments,
Lab represents the entire range of color that humans can see.
Lab is a rectangular coordinate system
with a central Lightness (L) axis.
This value is usually written as a unitless number;
for compatibility with the rest of CSS, it may also be written as a percentage.
100% means an L value of 100, not 1.0.
L=0% or 0 is deep black (no light at all)
while L=100% or 100 is a diffuse white.
Usefully, L=50% or 50 is mid gray, by design,
and equal increments in L are evenly spaced visually:
the Lab color space is intended to be
perceptually uniform
This figure shows, to the left, the Lightness axis of the CIE Lab color space.
Twenty-one neutral swatches are shown (L=0%, L=5%, to L=100%).
The steps are equally spaced, visually.
To the right, the same number of steps in
luminance
are equally spaced in light energy but
not
equally spaced visually.
The a and b axes convey hue;
positive values along the a axis are a purplish red
while negative values are the complementary color, a green.
Similarly, positive values along the b axis are yellow
and negative are the complementary blue/violet.
Desaturated colors have small values of a and b
and are close to the L axis;
saturated colors lie far from the L axis.
The illuminant is
D50
white, a standardized daylight spectrum with a color temperature of 5000K,
as reflected by a perfect diffuse reflector; it approximates the color of sunlight on a sunny day.
D50 is also the whitepoint used for the profile connection space in ICC color interconversion,
the whitepoint used in image editors which offer Lab editing,
and the value used by physical measurement devices
such as spectrophotometers and spectroradiometers,
when they report measured colors in Lab.
Conversion from colors specified using other white points is called a
chromatic adaptation transform
which models the changes in the human visual system as we adapt to a new lighting condition.
The linear Bradford algorithm
[ICC]
(a simplification of the original Bradford algorithm
[Bradford-CAT]
is the industry standard chromatic adaptation transform,
and is easy to calculate as it is a simple matrix multiplication.
CIE LCH has the same L axis as Lab,
but uses polar coordinates C (chroma) and H (hue),
making it a polar, cylindrical coordinate system.
C is the geometric distance from the L axis
and H is the angle from the positive a axis,
towards the positive b axis.
This figure shows the L=50 plane of the CIE Lab color space.
20 degree increments in CIE LCH are displayed as circles
at three levels of Chroma: 20, 40 and 60.
All the 20 Chroma colors fit inside sRGB gamut,
some of 40 and 60 Chroma are outside.
These out of gamut colors are visualized as grey, with a red warning outer stroke.
Note: The L axis in Lab and LCH
is not to be confused with the L axis in HSL.
For example, in HSL, the sRGB colors blue (#00F) and yellow (#FF0)
have the same value of L (50%) even though visually, blue is much darker.
This is much clearer in Lab:
sRGB blue is lab(29.567% 68.298 -112.0294)
while
sRGB yellow is lab(97.607% -15.753 93.388).
In Lab and LCH, if two colors have the same measured L value,
they have identical visual lightness.
HSL and related polar RGB models were developed
in an attempt
to give similar usability benefits for RGB that LCH gave to Lab,
but are significantly less accurate.
Although the use of CIE Lab and LCH is widespread,
it is known to have some problems. In particular:
Hue linearity
In the blue region (LCH Hue between 270° and 330°),
visual hue departs from what LCH predicts.
Plotting a set of blues of the same hue and differing Chroma,
which should lie on a straight line from the neutral axis,
instead form a curve.
Put another way,
as a saturated blue has it’s Chroma progressively reduced,
it becomes noticeably purple.
Hue uniformity
While hues in LCH are in general evenly spaced,
(and far better than HSL or HWB),
uniformity is not perfect.
Over-prediction of high Chroma differences
For high Chroma colors,
changes in Chroma are less noticeable
than for more neutral colors.
These deficiencies affect, for example,
creation of evenly spaced gradients,
gamut mapping from one color space to a smaller one,
and computation of the visual difference between two colors.
To compensate for this,
formulae to predict the visual difference between two colors
(delta E)
have been made more accurate over time
(but also, much more complex to compute).
The current industry standard formula,
delta E 2000,
works well to mitigate some of the Lab and LCH problems.
A sample implementation is given in
§ 20.1 ΔE2000
This does not help with hue curvature, however.
9.2.
Oklab and OkLCh
This section is not normative.
Tests
This section is not normative, it does not need tests.
Recently, Oklab,
an improved Lab-like space has been developed
[Oklab]
The corresponding polar form is called OkLCh.
It was produced by numerical optimization
of a large dataset of visually similar colors,
and has improved hue linearity,
hue uniformity,
and chroma uniformity
compared to CIE LCH.
Like CIE Lab, there is a central lightness L axis
which is usually written as a unitless number in the range [0,1];
for compatibility with the rest of CSS,
it may be written as a percentage. 100% means an L value of 1.0.
L=0% or 0.0 is deep black (no light at all) while L=100% or 1.0 is a diffuse white.
Note:
Unlike CIE Lab, which assumes adaptation to the diffuse white,
Oklab assumes adaptation to the color being defined,
which is intended to make it scale invariant.
As with CIE Lab, the a and b axes convey hue;
positive values along the a axis are a purplish red
while negative values are the complementary color, a green.
Similarly, positive values along the b axis are yellow
and negative are the complementary blue/violet.
The illuminant is
D65
, the same white point
as most RGB color spaces.
OkLCh has the same L axis as Oklab,
but uses polar coordinates C (chroma) and H (hue).
Note:
Unlike CIE LCH, where Chroma can reach values of 200 or more,
OkLCh Chroma ranges to 0.5 or so.
The hue angles between CIE LCH and OkLCh are broadly similar,
but not identical.
A constant CIE LCH hue slice,
showing the sRGB gamut around primary blue.
A noticeable purpling is immediately evident.
A constant OkLCh hue slice,
showing the sRGB gamut around primary blue.
The visual hue remains constant.
Because Oklab is more perceptually uniform than CIE Lab,
the color difference is a straightforward distance in 3D space
(root sum of squares).
Although trivial,
a sample implementation is give in
§ 20.2 ΔEOK
9.3.
Specifying Lab and LCH: the
lab()
and
lch()
functional notations
CSS allows colors to be directly expressed in Lab and LCH.
lab
()
lab


none


none


none

none
Percentages
Allowed for L, a and b
Percent reference range
for L: 0% = 0.0, 100% = 100.0
for a and b: -100% = -125, 100% = 125
Tests
lab-001.html
(live test)
(source)
lab-002.html
(live test)
(source)
lab-003.html
(live test)
(source)
lab-004.html
(live test)
(source)
lab-005.html
(live test)
(source)
lab-006.html
(live test)
(source)
lab-007.html
(live test)
(source)
lab-008.html
(live test)
(source)
lab-l-over-100-1.html
(live test)
(source)
lab-l-over-100-2.html
(live test)
(source)
color-valid.html
(live test)
(source)
color-computed-lab.html
(live test)
(source)
color-invalid-lab.html
(live test)
(source)
color-valid-lab.html
(live test)
(source)
In
Lab
the first argument specifies the CIE Lightness, L.
This is a number between
0%
or 0
and
100%
or 100
Values less than
0%
or 0 must be clamped to
0%
at parsed-value time;
values greater than
100%
or 100 are clamped to
100%
at parsed-value time.
The second and third arguments are the distances along the "a" and "b" axes
in the Lab color space,
as described in the previous section.
These values are signed
(allow both positive and negative values)
and theoretically unbounded
(but in practice do not exceed ±160
for real-world colors).
There is an optional fourth

component,
separated by a slash,
representing the
alpha component
If the lightness of a Lab color (after clamping) is
0%
or
100%
the color will be displayed as black, or white, respectively
due to gamut mapping to the display.
lab
29.2345
39.3825
20.0664
);
lab
52.2345
40.1645
59.9971
);
lab
60.2345
-5.3654
58.956
);
lab
62.2345
-34.9638
47.7721
);
lab
67.5345
-8.6911
-41.6019
);
lab
29.69
44.888
-29.04
lch
()
lch


none


none

none

none
Percentages
Allowed for L and C
Percent reference range
for L: 0% = 0.0, 100% = 100.0
for C: 0% = 0, 100% = 150
Powerless hue ε
C <= 0.0015
Tests
lch-001.html
(live test)
(source)
lch-002.html
(live test)
(source)
lch-003.html
(live test)
(source)
lch-004.html
(live test)
(source)
lch-005.html
(live test)
(source)
lch-006.html
(live test)
(source)
lch-007.html
(live test)
(source)
lch-008.html
(live test)
(source)
lch-009.html
(live test)
(source)
lch-010.html
(live test)
(source)
lch-l-over-100-1.html
(live test)
(source)
lch-l-over-100-2.html
(live test)
(source)
color-valid.html
(live test)
(source)
In CIE
LCH
the first argument specifies the CIE Lightness L,
interpreted identically to the Lightness argument of
lab()
The second argument is the chroma C,
(roughly representing the "amount of color").
Its minimum useful value is
while its maximum is theoretically unbounded
(but in practice does not exceed
230
).
If the provided value is negative,
it is clamped to
at parsed-value time.
The third argument is the hue angle H.
It’s interpreted similarly to the

argument of
hsl()
but doesn’t map hues to angles in the same way
because they are evenly spaced perceptually.
Instead,
0deg
points along the positive "a" axis (toward purplish red),
(as does
360deg
720deg
, etc.);
90deg
points along the positive "b" axis (toward mustard yellow),
180deg
points along the negative "a" axis (toward greenish cyan),
and
270deg
points along the negative "b" axis (toward sky blue).
There is an optional fourth

component,
separated by a slash,
representing the
alpha component
If the chroma of an LCH color is
0%
the hue component is
powerless
If the lightness of an LCH color (after clamping) is
0%
or
100%
the color will be displayed as black, or white, respectively
due to gamut mapping to the display.
lch
29.2345
44.2
27
);
lch
52.2345
72.2
56.2
);
lch
60.2345
59.2
95.2
);
lch
62.2345
59.2
126.2
);
lch
67.5345
42.5
258.2
);
lch
29.69
45.553
327.1
There is no Web compatibility issue
with
lab
or
lch
', which are new in this level of the specification, and so
lab()
and
lch()
do
not
support a
legacy color syntax
that separates all of their arguments with commas.
Using commas inside these functions is an error.
9.4.
Specifying Oklab and OkLCh: the
oklab()
and
oklch()
functional notations
CSS allows colors to be directly expressed in Oklab and OkLCh.
oklab
()
oklab


none


none


none

none
Percentages
Allowed for L, a and b
Percent reference range
for L: 0% = 0.0, 100% = 1.0
for a and b: -100% = -0.4, 100% = 0.4
Tests
oklab-001.html
(live test)
(source)
oklab-002.html
(live test)
(source)
oklab-003.html
(live test)
(source)
oklab-004.html
(live test)
(source)
oklab-005.html
(live test)
(source)
oklab-006.html
(live test)
(source)
oklab-007.html
(live test)
(source)
oklab-008.html
(live test)
(source)
oklab-009.html
(live test)
(source)
oklab-l-almost-0.html
(live test)
(source)
oklab-l-almost-1.html
(live test)
(source)
oklab-l-over-1-1.html
(live test)
(source)
oklab-l-over-1-2.html
(live test)
(source)
color-valid.html
(live test)
(source)
In
Oklab
the first argument specifies the Oklab Lightness.
This is a number between
0%
or 0
and
100%
or 1.0.
Values less than
0%
or 0.0 must be clamped to
0%
at parsed-value time;
values greater than
100%
or 1.0 are clamped to
100%
at parsed-value time.
The second and third arguments are the distances along
the "a" and "b" axes
in the Oklab color space,
as described in the previous section.
These values are signed
(allow both positive and negative values)
and theoretically unbounded
(but in practice do not exceed ±0.5).
There is an optional fourth

component,
separated by a slash,
representing the
alpha component
If the lightness of an Oklab color is
0%
or 0,
or
100%
or 1.0,
the color will be displayed as black, or white, respectively
due to gamut mapping to the display.
oklab
40.101
0.1147
0.0453
);
oklab
59.686
0.1009
0.1192
);
oklab
0.65125
-0.0320
0.1274
);
oklab
66.016
-0.1084
0.1114
);
oklab
72.322
-0.0465
-0.1150
);
oklab
42.1
41
-25
oklch
()
oklch


none


none

none

none
Percentages
Allowed for L and C
Percent reference range
for L: 0% = 0.0, 100% = 1.0
for C: 0% = 0.0 100% = 0.4
Powerless hue ε
C <= 0.000004
Tests
oklch-001.html
(live test)
(source)
oklch-002.html
(live test)
(source)
oklch-003.html
(live test)
(source)
oklch-004.html
(live test)
(source)
oklch-005.html
(live test)
(source)
oklch-006.html
(live test)
(source)
oklch-007.html
(live test)
(source)
oklch-008.html
(live test)
(source)
oklch-009.html
(live test)
(source)
oklch-010.html
(live test)
(source)
oklch-011.html
(live test)
(source)
oklch-l-almost-0.html
(live test)
(source)
oklch-l-almost-1.html
(live test)
(source)
oklch-l-over-1-1.html
(live test)
(source)
oklch-l-over-1-2.html
(live test)
(source)
color-valid.html
(live test)
(source)
In
OkLCh
the first argument specifies the OkLCh Lightness L,
interpreted identically to the Lightness argument of
oklab()
The second argument is the chroma C.
Its minimum useful value is
while its maximum is theoretically unbounded
(but in practice does not exceed
0.5
).
If the provided value is negative,
it is clamped to
at parsed-value time.
The third argument is the hue angle H.
It’s interpreted similarly to the

arguments
of
hsl()
and
lch()
but doesn’t map hues to angles in the same way.
0deg
points along the positive "a" axis (toward purplish red),
(as does
360deg
720deg
, etc.);
90deg
points along the positive "b" axis (toward mustard yellow),
180deg
points along the negative "a" axis (toward greenish cyan),
and
270deg
points along the negative "b" axis (toward sky blue).
There is an optional fourth

component,
separated by a slash,
representing the
alpha component
If the chroma of an OkLCh color is
0%
or 0,
the hue component is
powerless
If the lightness of an OkLCh color is
0%
or 0,
or
100%
or 1.0,
the color will be displayed as black, or white, respectively
due to gamut mapping to the display.
oklch
40.101
0.12332
21.555
);
oklch
59.686
0.15619
49.7694
);
oklch
0.65125
0.13138
104.097
);
oklch
0.66016
0.15546
134.231
);
oklch
72.322
0.12403
247.996
);
oklch
42.1
48.25
328.4
There is no Web compatibility issue
with
oklab
or
oklch
', which are new in this level of the specification, and so
oklab()
and
oklch()
do
not
support a
legacy color syntax
that separates all of their arguments with commas.
Using commas inside these functions is an error.
9.5.
Converting Lab or Oklab colors to LCH or OkLCh colors
Conversion to the polar form is trivial:
C = sqrt(a^2 + b^2)
if (C > epsilon) H = atan2(b, a) else H is missing
L is the same
For extremely small values of a and b (near-zero Chroma),
although the visual color does not change from being on the neutral axis,
small changes to the values can result in the reported hue angle swinging about wildly
and being essentially random.
In CSS, this means the hue is
powerless
and treated as
missing
when converted into LCH or OkLCh;
in non-CSS contexts this might be reflected as a missing value, such as NaN.
9.6.
Converting LCH or OkLCh colors to Lab or Oklab colors
Conversion to the rectangular form is trivial:
If H is missing, a = b = 0
Otherwise,
a = C cos(H)
b = C sin(H)
L is the same
10.
Predefined Color Spaces
CSS provides several predefined color spaces
including
display-p3
[Display-P3]
which is a wide gamut space typical of current wide-gamut monitors,
prophoto-rgb
, widely used by photographers
and
rec2020
[Rec.2020]
which is a broadcast industry standard,
ultra-wide gamut space capable of representing almost all visible real-world colors.
10.1.
Specifying Predefined Colors: the
color()
function
The
color()
function allows a color to be specified
in a particular, specified
color space
(rather than the implicit sRGB color space that most of the other color functions operate in).
Its syntax is:
color
()
color


none







none

srgb
srgb-linear
display-p3
display-p3-linear
a98-rgb
prophoto-rgb
rec2020




none

xyz
xyz-d50
xyz-d65
Tests
color-computed-color-function.html
(live test)
(source)
color-invalid-color-function.html
(live test)
(source)
color-valid-color-function.html
(live test)
(source)
The color function takes parameters specifying a color, in an explicitly listed color space.
It represents either an
invalid color
, as described below,
or a
valid color
The parameters have the following form:
An

denoting
one of the
predefined color spaces
(such as
display-p3
Individual
predefined color spaces
may further restrict whether

s or

or both, may be used.
If the

names a non-existent color space
(a name that does not match one of the
predefined color spaces
),
this argument represents an
invalid color
The three parameter values that the color space takes (RGB or XYZ values).
An out of gamut color has component values
less than 0 or 0%, or greater than 1 or 100%.
These are not invalid, and are retained for intermediate computations;
instead, for display, they are
css gamut mapped
using a relative colorimetric intent
which brings the values
(in the display color space)
within the range 0/0% to 1/100%
at actual-value time.
An optional slash-separated

Tests
color-valid.html
(live test)
(source)
There is no Web compatibility issue
with
color()
, which is new in this level of the specification, and so
color()
does
not
support a
legacy color syntax
that separates all of its arguments with commas.
Using commas inside this function is an error.
A color which is either an
invalid color
or
an
out of gamut
color
can’t be displayed
If the specified color
can be displayed
(that is, it isn’t an
invalid color
and isn’t
out of gamut
then this is the actual value of the
color()
function.
If the specified color
is a
valid color
but
can’t be displayed
the actual value is derived from the specified color,
css gamut mapped
for display.
If the color is an
invalid color
the used value is
opaque black
This very intense lime color is in-gamut for rec.2020:
color
rec2020
0.42053
0.979780
0.00579
);
in LCH, that color is
lch
85.9017
166.116
138.207
);
in display-p3, that color is
color
display-p3
-0.350289
1.00707
-0.144209
);
and is out of gamut for display-p3
(red and blue are negative, green is greater than 1).
If you have a display-p3 screen, that color is:
valid
in gamut
(for rec.2020)
out of gamut
(for your display)
and so
can’t be displayed
The color used for display will be a less intense color
produced automatically by gamut mapping.
This example has a typo!
An intense green is provided in profoto-rgb space (which doesn’t exist).
This makes it invalid, so the used value is
opaque black
color
profoto-rgb
0.4835
0.9167
0.2188
10.2.
The Predefined sRGB Color Space: the
sRGB
keyword
The
sRGB
predefined color space
defined below
is the same as is used for legacy sRGB colors,
such as
rgb()
srgb
The
srgb
[SRGB]
color space accepts three numeric parameters,
representing the red, green, and blue components of the color.
In-gamut colors have all three components in the range [0, 1].
The whitepoint is
D65
[SRGB]
specifies two viewing conditions,
encoding
and
typical
. The
[ICC]
recommends using the
encoding
conditions for color conversion and for optimal viewing, which are
the values in the table below.
sRGB is the default color space for CSS,
used for all the legacy color functions.
It has the following characteristics:
Red chromaticity
0.640
0.330
Green chromaticity
0.300
0.600
Blue chromaticity
0.150
0.060
White chromaticity
D65
Transfer function
see below
White luminance
80.0 cd/m
Black luminance
0.20 cd/m
Image state
display-referred
Percentages
Allowed for R, G and B
Percent reference range
for R,G,B: 0% = 0.0, 100% = 1.0
let
sign
let
abs
Math
abs
);
if
abs
<=
0.04045
cl
12.92
else
cl
sign
Math
pow
((
abs
0.055
1.055
2.4
));
c is the gamma-encoded red, green or blue component.
cl is the corresponding linear-light component.
Visualization of the sRGB color space in LCH. The primaries and secondaries are shown.
Tests
predefined-001.html
(live test)
(source)
predefined-002.html
(live test)
(source)
color-valid.html
(live test)
(source)
10.3.
The Predefined Linear-Light sRGB Color Space: the
srgb-linear
keyword
The
sRGB-linear
predefined color space
is the same as
srgb
except
that the transfer function
is linear-light (there is no gamma-encoding).
srgb-linear
The
srgb-linear
[SRGB]
color space accepts three numeric parameters,
representing the red, green, and blue components of the color.
In-gamut colors have all three components in the range [0, 1].
The whitepoint is
D65
It has the following characteristics:
Red chromaticity
0.640
0.330
Green chromaticity
0.300
0.600
Blue chromaticity
0.150
0.060
White chromaticity
D65
Transfer function
unity, see below
White luminance
80.0 cd/m
Black luminance
0.20 cd/m
Image state
display-referred
Percentages
Allowed for R, G and B
Percent reference range
for R,G,B: 0% = 0.0, 100% = 1.0
cl
c is the red, green or blue component.
cl is the corresponding linear-light component, which is identical.
To avoid banding artifacts, a
higher precision is required
for
srgb-linear
than for
srgb
For example, these are the same color
color
srgb
0.691
0.139
0.259
color
srgb-linear
0.435
0.017
0.055
Tests
srgb-linear-001.html
(live test)
(source)
srgb-linear-002.html
(live test)
(source)
srgb-linear-003.html
(live test)
(source)
srgb-linear-004.html
(live test)
(source)
color-valid.html
(live test)
(source)
10.4.
The Predefined Display P3 Color Space: the
display-p3
keyword
display-p3
The
display-p3
[Display-P3]
color space accepts three numeric parameters,
representing the red, green, and blue components of the color.
In-gamut colors have all three components in the range [0, 1].
It uses the same primary chromaticities as
[DCI-P3]
but with a
D65
whitepoint, and the same transfer curve as sRGB.
Modern displays, TVs, laptop screens and phone screens
are able to display all, or nearly all,
of the display-p3 gamut.
It has the following characteristics:
Red chromaticity
0.680
0.320
Green chromaticity
0.265
0.690
Blue chromaticity
0.150
0.060
White chromaticity
D65
Transfer function
same as srgb
White luminance
80.0 cd/m
Black luminance
0.80 cd/m
Image state
display-referred
Percentages
Allowed for R, G and B
Percent reference range
for R,G,B: 0% = 0.0, 100% = 1.0
Visualization of the P3 color space in LCH.
The primaries and secondaries are shown
(but in sRGB, not in the correct colors).
For comparison, the sRGB primaries and secondaries
are also shown, as dashed circles.
P3 primaries have higher Chroma.
Tests
predefined-005.html
(live test)
(source)
predefined-006.html
(live test)
(source)
display-p3-001.html
(live test)
(source)
display-p3-002.html
(live test)
(source)
display-p3-003.html
(live test)
(source)
display-p3-004.html
(live test)
(source)
display-p3-005.html
(live test)
(source)
display-p3-006.html
(live test)
(source)
2d.color.space.p3.fillText.html
(live test)
(source)
2d.color.space.p3.fillText.shadow.html
(live test)
(source)
2d.color.space.p3.strokeText.html
(live test)
(source)
2d.color.space.p3.to.p3.html
(live test)
(source)
2d.color.space.p3.to.srgb.html
(live test)
(source)
2d.color.space.p3.toBlob.p3.canvas.html
(live test)
(source)
2d.color.space.p3.toBlob.with.putImageData.html
(live test)
(source)
2d.color.space.p3.toDataURL.jpeg.p3.canvas.html
(live test)
(source)
2d.color.space.p3.toDataURL.p3.canvas.html
(live test)
(source)
2d.color.space.p3.toDataURL.with.putImageData.html
(live test)
(source)
10.5.
The Predefined Linear-Light Display P3 Color Space: the
display-p3-linear
keyword
The
display-p3-linear
predefined color space
is the same as
display-p3
except
that the transfer function
is linear-light (there is no gamma-encoding).
It has the following characteristics:
Red chromaticity
0.680
0.320
Green chromaticity
0.265
0.690
Blue chromaticity
0.150
0.060
White chromaticity
D65
Transfer function
unity, see below
White luminance
80.0 cd/m
Black luminance
0.80 cd/m
Image state
display-referred
Percentages
Allowed for R, G and B
Percent reference range
for R,G,B: 0% = 0.0, 100% = 1.0
cl
c is the red, green or blue component.
cl is the corresponding linear-light component, which is identical.
To avoid banding artifacts, a
higher precision is required
for
display-p3-linear
than for
display-p3
For example, these are the same color
color
display-p3
0.591
0.123
0.264
color
display-p3-linear
0.3081
0.014
0.0567
Tests
display-p3-linear-001.html
(live test)
(source)
display-p3-linear-002.html
(live test)
(source)
display-p3-linear-003.html
(live test)
(source)
display-p3-linear-004.html
(live test)
(source)
display-p3-linear-005.html
(live test)
(source)
display-p3-linear-006.html
(live test)
(source)
10.6.
The Predefined A98 RGB Color Space: the
a98-rgb
keyword
a98-rgb
The
a98-rgb
color space accepts three numeric parameters,
representing the red, green, and blue components of the color.
In-gamut colors have all three components in the range [0, 1].
The transfer curve is
a gamma function, close to but not exactly 1/2.2.
It has the following characteristics:
Red chromaticity
0.6400
0.3300
Green chromaticity
0.2100
0.7100
Blue chromaticity
0.1500
0.0600
White chromaticity
D65
Transfer function
256/563
White luminance
160.0 cd/m
Black luminance
0.5557 cd/m
Image state
display-referred
Percentages
Allowed for R, G and B
Percent reference range
for R,G,B: 0% = 0.0, 100% = 1.0
Visualization of the A98 color space in LCH.
The primaries and secondaries are shown
(but in sRGB, not in the correct colors).
For comparison, the sRGB primaries and secondaries
are also shown, as dashed circles.
a98 primaries have higher Chroma,
especially the yellow, green and cyan.
Tests
predefined-007.html
(live test)
(source)
predefined-008.html
(live test)
(source)
a98rgb-001.html
(live test)
(source)
a98rgb-002.html
(live test)
(source)
a98rgb-003.html
(live test)
(source)
a98rgb-004.html
(live test)
(source)
color-valid.html
(live test)
(source)
10.7.
The Predefined ProPhoto RGB Color Space: the
prophoto-rgb
keyword
prophoto-rgb
The
prophoto-rgb
color space accepts three numeric parameters,
representing the red, green, and blue components of the color.
In-gamut colors have all three components in the range [0, 1].
The transfer curve is
a gamma function with a value of 1/1.8,
and a small linear portion near black.
The white point is
D50
, the same as is used by CIE Lab. Thus,
conversion to CIE Lab does not require the chromatic adaptation step.
The ProPhoto RGB space uses hyper-saturated,
non physically realizable primaries.
These were chosen to allow
a wide color gamut and in particular,
to minimize hue shifts under tonal manipulation.
It is often used in digital photography as a wide gamut
color space for the archival version of
photographic images. The
prophoto-rgb
color space allows CSS to
specify colors that will match colors in such images
having the same RGB values.
The ProPhoto RGB space was originally developed by Kodak
and is described in
[Wolfe]
It was standardized by ISO as
[ROMM]
[ROMM-RGB]
The white luminance is given as a range, and
the viewing flare (and thus, the black luminance)
is 0.5% to 1.0% of this.
It has the following characteristics:
Red chromaticity
0.734699
0.265301
Green chromaticity
0.159597
0.840403
Blue chromaticity
0.036598
0.000105
White chromaticity
D50
Transfer function
see below
White luminance
160.0 to 640.0 cd/m
Black luminance
See text
Image state
display-referred
Percentages
Allowed for R, G and B
Percent reference range
for R,G,B: 0% = 0.0, 100% = 1.0
const
16
512
let
sign
let
abs
Math
abs
);
if
abs
<=
cl
16
else
cl
sign
Math
pow
1.8
);
c is the gamma-encoded red, green or blue component.
cl is the corresponding linear-light component.
Visualization of the prophoto-rgb color space in LCH. The primaries and secondaries are shown
(but in sRGB, not in the correct colors).
For comparison, the sRGB primaries and secondaries
are also shown, as dashed circles.
prophoto-rgb primaries and secondaries have much higher Chroma,
but much of this ultrawide gamut
does not correspond to physically realizable colors.
Tests
predefined-009.html
(live test)
(source)
predefined-010.html
(live test)
(source)
prophoto-rgb-001.html
(live test)
(source)
prophoto-rgb-002.html
(live test)
(source)
prophoto-rgb-003.html
(live test)
(source)
prophoto-rgb-004.html
(live test)
(source)
prophoto-rgb-005.html
(live test)
(source)
color-valid.html
(live test)
(source)
10.8.
The Predefined ITU-R BT.2020-2 Color Space: the
rec2020
keyword
rec2020
The
rec2020
[Rec.2020]
color space accepts three numeric parameters,
representing the red, green, and blue components of the color.
In-gamut colors have all three components in the range [0, 1],
("full-range", in video terminology).
ITU Reference 2020 is used for
Ultra High Definition, 4k and 8k television.
The primaries are physically realizable,
but with difficulty
as they lie very close to the spectral locus.
Current displays are unable to reproduce the full gamut of rec2020.
Coverage is expected to increase over time as displays improve.
It has the following characteristics:
Red chromaticity
0.708
0.292
Green chromaticity
0.170
0.797
Blue chromaticity
0.131
0.046
White chromaticity
D65
Transfer function
gamma 2.40, from
[REC_BT.1886]
Image state
display-referred
Percentages
Allowed for R, G and B
Percent reference range
for R,G,B: 0% = 0.0, 100% = 1.0
Visualization of the rec2020 color space in LCH. The primaries and secondaries are shown
(but in sRGB, not in the correct colors).
For comparison, the sRGB primaries and secondaries
are also shown, as dashed circles.
rec2020 primaries have much higher Chroma.
Tests
predefined-011.html
(live test)
(source)
predefined-012.html
(live test)
(source)
rec2020-001.html
(live test)
(source)
rec2020-002.html
(live test)
(source)
rec2020-003.html
(live test)
(source)
rec2020-004.html
(live test)
(source)
rec2020-005.html
(live test)
(source)
color-valid.html
(live test)
(source)
10.9.
The Predefined CIE XYZ Color Spaces: the
xyz-d50
xyz-d65
, and
xyz
keywords
xyz-d50
xyz-d65
xyz
The
xyz
color space accepts three numeric parameters,
representing the X,Y and Z values.
It represents the CIE XYZ
[COLORIMETRY]
color space,
scaled such that diffuse white has a
luminance
(Y) of 1.0.
and, if necessary, chromatically adapted to the reference white.
The reference white for
xyz-d50
is
D50
, while
the reference white for
xyz-d65
and
xyz
is
D65
Values greater than 1.0/100% are allowed and must not be clamped;
colors where Y is greater than 1.0 represent colors brighter than diffuse white.
Values less than 0/0% are uncommon,
but can occur as a result of chromatic adaptation,
and likewise must not be clamped.
It has the following characteristics:
Percentages
Allowed for X,Y,Z
Percent reference range
for X,Y,Z: 0% = 0.0, 100% = 1.0
These are exactly equivalent:
#7654CD
rgb
46.27
32.94
80.39
lab
44.36
36.05
-58.99
color
xyz-d50
0.2005
0.14089
0.4472
color
xyz-d65
0.21661
0.14602
0.59452
These colors are exactly equivalent, and represent white:
#FFFFFF
color
xyz-d50
0.9643
0.8251
color
xyz-d65
0.9505
1.089
Tests
predefined-016.html
(live test)
(source)
xyz-001.html
(live test)
(source)
xyz-002.html
(live test)
(source)
xyz-003.html
(live test)
(source)
xyz-004.html
(live test)
(source)
xyz-005.html
(live test)
(source)
xyz-d50-001.html
(live test)
(source)
xyz-d50-002.html
(live test)
(source)
xyz-d50-003.html
(live test)
(source)
xyz-d50-004.html
(live test)
(source)
xyz-d50-005.html
(live test)
(source)
xyz-d65-001.html
(live test)
(source)
xyz-d65-002.html
(live test)
(source)
xyz-d65-003.html
(live test)
(source)
xyz-d65-004.html
(live test)
(source)
xyz-d65-005.html
(live test)
(source)
color-valid.html
(live test)
(source)
10.10.
Converting Predefined Color Spaces to Lab or Oklab
For all predefined RGB color spaces,
conversion to Lab requires several steps,
although in practice all but the first step are linear calculations and can be combined.
Convert from gamma-encoded RGB to linear-light RGB (undo gamma encoding)
Convert from linear RGB to CIE XYZ
If needed, convert from a
D65
whitepoint
(used by
sRGB
display-p3
a98-rgb
and
rec2020
to the
D50
whitepoint used in Lab,
with the linear Bradford transform.
prophoto-rgb
already has a
D50
whitepoint.
Convert D50-adapted XYZ to Lab
Conversion to Oklab is similar,
but the chromatic adaptation step
is only needed for
prophoto-rgb
Convert from gamma-encoded RGB to linear-light RGB (undo gamma encoding)
Convert from linear RGB to CIE XYZ
If needed, convert from a
D50
whitepoint (used by
prophoto-rgb
to the
D65
whitepoint used in Oklab,
with the linear Bradford transform.
Convert D65-adapted XYZ to Oklab
There is sample JavaScript code for these conversions
in
§ 19 Sample code for Color Conversions
10.11.
Converting Lab or Oklab to Predefined RGB Color Spaces
Conversion from Lab to predefined spaces like
display-p3
or
rec2020
also requires multiple steps,
and again in practice all but the last step are linear calculations and can be combined.
Convert Lab to (D50-adapted) XYZ
If needed, convert from a
D50
whitepoint (used by Lab)
to the
D65
whitepoint used in sRGB and most other RGB spaces,
with the linear Bradford transform.
prophoto-rgb
' does not require this step.
Convert from (D65-adapted) CIE XYZ to linear RGB
Convert from linear-light RGB to RGB (do gamma encoding)
Conversion from Oklab is similar,
but the chromatic adaptation step
is only needed for
prophoto-rgb
Convert Oklab to (D65-adapted) XYZ
If needed, convert from a
D65
whitepoint (used by Oklab)
to the
D50
whitepoint used in
prophoto-rgb
with the linear Bradford transform.
Convert from (D65-adapted) CIE XYZ to linear RGB
Convert from linear-light RGB to RGB (do gamma encoding)
There is sample JavaScript code for these conversions
in
§ 19 Sample code for Color Conversions
Implementations may choose to implement these steps in some other way
(for example, using an ICC profile with relative colorimetric rendering intent)
provided the results are the same for colors inside both the source and destination gamuts.
10.12.
Converting Between Predefined RGB Color Spaces
Conversion from one predefined RGB color space to another
requires multiple steps,
one of which is only needed when the whitepoints differ.
To convert from
src
to
dest
Convert from gamma-encoded
src
RGB to linear-light
src
RGB (undo gamma encoding)
Convert from linear
src
RGB to CIE XYZ
If
src
and
dest
have different whitepoints,
convert the XYZ value from
src
White to
dest
White
with the linear Bradford transform.
Convert from CIE XYZ to linear
dest
RGB
Convert from linear-light
dest
RGB to
dest
RGB (do gamma encoding)
There is sample JavaScript code for this conversion
for the predefined RGB color spaces, in
§ 19 Sample code for Color Conversions
10.13.
Simple Alpha Compositing
When drawing, implementations must handle alpha
according to the rules in
Section 5.1 Simple alpha compositing
of
[Compositing]
11.
Converting Colors
Tests
This section provides algorithms used later, it does not need tests.
Colors may be converted
from one color space to another and,
provided that there is no gamut mapping
and that each color space can represent out of gamut colors,
(for RGB spaces, this means that the transfer function is defined over the extended range)
then (subject to numerical precision and round-off error)
the two colors will look the same and represent the same color sensation.
To
prepare a color
col1
for conversion
Change any
powerless component
s in
src
to
missing component
If
src
is in a
cylindrical polar color
representation,
convert
col1
to the corresponding
rectangular orthogonal color
representation
and let this be the new
col1
To
convert a color
col1
in a source color space
src
with white point
src-white
to a color
col2
in destination color space
dest
with white point
dest-white
where
src
and
dest
are
different
prepare
col1
for conversion
Replace any
missing component
with zero.
If
src
is not a linear-light representation,
convert it to linear light (undo gamma-encoding)
and let this be the new
col1
Convert
col1
to CIE XYZ with a given whitepoint
src-white
and let this be
xyz
If
dest-white
is not the same as
src-white
chromatically adapt
xyz
to
dest-white
using a linear Bradford
chromatic adaptation transform
and let this be the new
xyz
If
dest
is a
cylindrical polar color
representation,
let
dest-rect
be the corresponding
rectangular orthogonal color
representation.
Otherwise, let
dest-rect
be
dest
Convert
xyz
to
dest
followed by applying any transfer function (gamma encoding),
producing
col2
If
dest
is a physical output color space, such as a display,
then
col2
must be
css gamut mapped
so that it
can be displayed
If
dest-rect
is not the same as
dest
in other words
dest
is a
cylindrical polar color
representation,
convert from
dest-rect
to
dest
, and let this be
col2
This may produce
missing component
s.
12.
Comparing

Values
Two

values are
equivalent colors
when they compare as equal using the algorithm below.
This comparison is used,
for example,
by
style()
container queries
[CSS-CONDITIONAL-5]
and by CSS Transitions
[CSS-TRANSITIONS-1]
to determine whether a color value has changed.
Given two

values
C1
and
C2
they are
equivalent colors
if and only if
the following algorithm returns true:
For each of
C1
and
C2
convert any
powerless component
s to
missing component
s.
If
C1
and
C2
share the same

Compare their components one by one,
including the alpha channel.
missing component
is only equal to
another
missing component
Two numeric components are considered equal
if they differ by no more than a small
implementation-defined ε.
Return true if and only if
all components compare as equal.
Otherwise,
C1
and
C2
are in different

s.
If either color has at least one
missing component
return false.
Otherwise, neither color has any
missing component
Convert both
C1
and
C2
to
oklab
then return true if and only if
all components (including alpha) of the converted colors
compare as equal,
using an implementation-defined ε as in step 2.
Note:
Two colors that are expressed in different

but are colorimetrically identical—for example,
red
and
color(srgb 1 0 0)
—are
equivalent colors
by step 4 of this algorithm,
since they convert to the same
oklab
value.
For the purposes of this comparison,
rgb()
rgba()
hsl()
hsla()
hwb()
hex colors
named colors
, and
system colors
are all considered to be in the
srgb

Tests
query-style-color.html
(live test)
(source)
13.
Color Interpolation
Color interpolation happens with
gradients,
compositing,
filters,
transitions,
animations, and
color mixing and color modification functions.
Interpolation between two

values
takes place by executing the following steps:
checking the two colors for
analogous components
and
analogous sets
which will be
carried forward
prepare both colors for conversion.
this changes any
powerless
components to
missing
values
converting them both to a given color space
which will be referred to as the
interpolation color space
below.
(if required) re-inserting
carried forward
values in the converted colors
(if required) fixing up the hues, depending on the selected

changing the color components to
premultiplied
form
linearly interpolating each component of the computed value of the color separately
undoing
premultiplication
Interpolating to or from
currentcolor
is possible.
The numerical value used for this purpose is the used value.
13.1.
Color Space for Interpolation
Various features in CSS depend on interpolating colors.
Examples include:

filter
animation
transition
color-mix()
relative color
syntax
Mixing or otherwise combining colors
has different results depending on the
interpolation color space
used.
Thus, different color spaces may be more appropriate for each interpolation use case.
In some cases, the result of physically mixing two colored lights is desired.
In that case, the CIE
XYZ
display-p3-linear
or
srgb-linear
color spaces are appropriate, because they are linear in light intensity.
If colors need to be evenly spaced perceptually (such as in a gradient),
the
Oklab
color space (and the older
Lab
), are designed to be perceptually uniform.
If avoiding graying out in color mixing is desired, i.e. maximizing chroma throughout the transition,
OkLCh
(and the older
LCH
) work well for that.
Lastly, compatibility with legacy Web content may be the most important consideration.
The
sRGB
color space, which is neither linear-light nor perceptually uniform, is the choice here,
even though it produces poorer results (overly dark or greyish mixes).
These features are collectively termed the
host syntax
To permit a host syntax to indicate the
interpolation color space
this specification exports a
color-interpolation-method
production.
It is not used by this specification itself,
only exposed so that other specifications can use it;
see e.g. use in
CSS Images 4
§ 3.1 Linear Gradients: the linear-gradient() notation
The host syntax should define
what the
default
interpolation color space
should be for each case,
and preferably provide syntax for authors to override this default.
If such syntax is part of a property value, it should use the
color-interpolation-method
production,
defined below for easy reference from other specifications.
This ensures consistency across CSS,
and that further customizations on how color interpolation is performed
can automatically percolate across all of CSS.

=


srgb
srgb-linear
display-p3
display-p3-linear
a98-rgb
prophoto-rgb
rec2020
lab
oklab


hsl
hwb
lch
oklch

shorter
longer
increasing
decreasing
hue

= in



The keywords in the definitions of

and

each refer to their corresponding color space,
represented in CSS either by the functional syntax with the same name,
or (if no such function is present), by the corresponding

in the
color()
function.
Tests
color-mix-percents-01.html
(live test)
(source)
color-mix-percents-02.html
(live test)
(source)
gradients-with-transparent.html
(live test)
(source)
gradient-eval-001.html
(live test)
(source)
gradient-eval-002.html
(live test)
(source)
gradient-eval-003.html
(live test)
(source)
gradient-eval-004.html
(live test)
(source)
gradient-eval-005.html
(live test)
(source)
gradient-eval-006.html
(live test)
(source)
gradient-eval-007.html
(live test)
(source)
gradient-eval-008.html
(live test)
(source)
gradient-eval-009.html
(live test)
(source)
gradient-none-interpolation.html
(live test)
(source)
oklab-gradient.html
(live test)
(source)
srgb-gradient.html
(live test)
(source)
srgb-linear-gradient.html
(live test)
(source)
xyz-gradient.html
(live test)
(source)
gradient-interpolation-method-valid.html
(live test)
(source)
gradient-interpolation-method-invalid.html
(live test)
(source)
gradient-interpolation-method-computed.html
(live test)
(source)
If the host syntax does not define what color space
interpolation should take place in,
it defaults to Oklab.
For a

if the

is not specified,
it defaults to
shorter
However, user agents
must
handle interpolation
between legacy sRGB color formats
(hex colors, named colors,
rgb()
hsl()
or
hwb()
and the equivalent alpha-including forms)
in gamma-encoded sRGB space.
This provides Web compatibility;
legacy sRGB content interpolates in the sRGB space by default.
Tests
legacy-color-gradient.html
(live test)
(source)
This also means that authors can choose
to opt-in to better interpolation,
even between sRGB colors,
by using the non-legacy
color(srgb r g b)
form
for at least one of their colors,
or by explicitly specifying an
interpolation color space
Tests
css-color-4-colors-default-to-oklab-gradient.html
(live test)
(source)
If the colors to be interpolated are outside the gamut
of the
interpolation color space
then once converted to that space,
they will contain out of range values.
These are not clipped; the values must be interpolated as-is.
13.2.
Interpolating with Missing Components
In the course of converting the two colors
to the
interpolation color space
any
missing components
would be replaced with the value 0.
Thus, the first stage in interpolating two colors
is to classify any
missing components
in the input colors,
and compare them to the components of the
interpolation color space
If any
analogous components
which are
missing components
are found,
they will be
carried forward
and re-inserted in the converted color
before premultiplication, and
before linear interpolation takes place.
Similarly, if every component of an
analogous set
(defined below)
in the original color
is a
missing component
they are all
carried forward
and re-inserted
in the corresponding
analogous set
of the
interpolation color space
The
analogous components
are as follows:
Category
Components
Reds
r,x
Greens
g,y
Blues
b,z
Lightness
Colorfulness
C, S
Hue
Opponent a
Opponent b
Alpha
alpha
Note:
for the purposes of this classification,
the XYZ spaces are considered super-saturated RGB spaces.
Also, despite Saturation being Lightness-dependent,
it falls in the same category as Chroma here.
The Whiteness and Blackness components of HWB
have no analogs in other color spaces.
Additionally, for any two color spaces,
the components that remain
after removing all
analogous components
form an
analogous set
of components.
Note:
Because the full set of all color components
is the
analogous set
that remains
when there are no individual
analogous components
a color with all color components
missing
will have all color components
missing
in the
interpolation color space
as well.
When converting
lab
50
none none
to LCH
for interpolation,
Lightness is individually
analogous
The remaining components
and
in Lab;
and
in LCH)
form an
analogous set
Since both
and
are
missing
both
and
are carried forward as
missing
giving
lch
50
none none
rather than
lch
50
Similarly,
rgb
none none none /
50
converted to OKLab for interpolation
yields
oklab
none none none /
50
because the three color components
form the
analogous set
(there are no individual
analogous components
between sRGB and OKLab).
Tests
gradient-none-interpolation.html
(live test)
(source)
For example, if these two colors
are to be interpolated in OkLCh,
the missing hue in the CIE LCH color
is analogous to the hue component of OkLCh
and will be carried forward
while the missing blue component
in the second color
is not analogous to any OkLCh component
and will not be carried forward:
lch
50
0.02
none
color
display-p3
0.7
0.5
none
which convert to
oklch
56.897
0.0001
oklch
63.612
0.1522
78.748
and with carried forward
missing component
re-inserted,
the two colors to be interpolated are:
oklch
56.897
0.0001
none
oklch
63.612
0.1522
78.748
If a color with a carried forward
missing component
is interpolated
with another color
which is not missing that component,
the
missing component
is treated as having
the
other color’s
component value.
Therefore,
the carrying-forward step
must be performed
before
any
powerless component
handling.
For example, if these two colors are interpolated,
the second of which has a missing hue:
oklch
78.3
0.108
326.5
oklch
39.2
0.4
none
Then the actual colors to be interpolated are
oklch
78.3
0.108
326.5
oklch
39.2
0.4
326.5
and not
oklch
78.3
0.108
326.5
oklch
39.2
0.4
If the carried forward
missing component
is alpha, the color must be
premultiplied
with this carried forward value,
not with the zero value that would have resulted from color conversion.
For example, if these two colors are interpolated,
the second of which has a missing alpha:
oklch
0.783
0.108
326.5
0.5
oklch
0.392
0.4
/ none
Then the actual colors to be interpolated are
oklch
78.3
0.108
326.5
0.5
oklch
39.2
0.4
0.5
giving the premultiplied OkLCh values [0.3915, 0.054, 326] and [0.196, 0.2, 0].
If both colors are
missing
a given component,
the interpolated color
will also be
missing
that component.
13.3.
Interpolating with Alpha
When the colors to be interpolated are not fully opaque,
they are first
premultiplied
as follows:
If the alpha value is
none
, the premultiplied value is the un-premultiplied value. Otherwise,
If any component value is
none
, the premultiplied value is also
none
For
rectangular orthogonal color
coordinate systems, all component values are multiplied by the alpha value.
For
cylindrical polar color
coordinate systems, the hue angle is
not
premultiplied, but the other two axes
are
premultiplied.
To obtain a color value from a premultiplied color value,
If the interpolated alpha value is zero or
none
the un-premultiplied value is the premultiplied value. Otherwise,
If any component value is
none
the un-premultiplied value is also
none
otherwise, each component which had been premultiplied
is divided by the interpolated alpha value.
Tests
color-transition-premultiplied.html
(live test)
(source)
Why is premultiplied alpha useful?
Interpolating colors using the premultiplied representations
tends to produce more attractive transitions than the non-premultiplied representations,
particularly when transitioning from a fully opaque color to fully transparent.
Note that transitions where either the transparency or the color are held constant
(for example, transitioning between
rgba
255
100
(opaque red)
and
rgba
255
100
(opaque blue),
or
rgba
255
100
(opaque red)
and
rgba
255
(transparent red))
have identical results whether the color interpolation is done in premultiplied or non-premultiplied color-space.
Differences only arise when
both
the color and transparency differ between the two endpoints.
The following example illustrates the difference between
a gradient transitioning via pre-multiplied values
(in this case sRGB, since all colors involved are legacy colors)
and one transitioning (incorrectly) via non-premultiplied values.
In both of these examples,
the gradient is drawn over a white background.
Both gradients could be written with the following value:
linear-gradient
90
deg
red
transparent
blue
With premultiplied colors,
transitions to or from "transparent" always look nice:
(Image requires SVG)
On the other hand,
if a gradient were to incorrectly transition in non-premultiplied space,
the center of the gradient would be a noticeably grayish color,
because "transparent" is actually a shorthand for
rgba(0,0,0,0)
, or transparent black,
meaning that the red transitions to a black
as it loses opacity,
and similarly with the blue’s transition:
(Image requires SVG)
For example, to interpolate, in the sRGB color space, the two sRGB colors
rgb(24% 12% 98% / 0.4)
and
rgb(62% 26% 64% / 0.6)
they would first be converted to premultiplied form
[9.6% 4.8% 39.2% ]
and
[37.2% 15.6% 38.4%]
before interpolation.
The midpoint of linearly interpolating these colors
would be [23.4% 10.2% 38.8%]
which, with an alpha value of 0.5,
is
rgb(46.8% 20.4% 77.6% / 0.5)
when premultiplication is undone.
To interpolate, in the Lab color space, the two colors
rgb(76% 62% 03% / 0.4)
and
color(display-p3 0.84 0.19 0.72 / 0.6)
they are first converted to lab
lab(66.927% 4.873 68.622 / 0.4)
lab(53.503% 82.672 -33.901 / 0.6)
then the L, a and b coordinates are premultiplied before interpolation
[26.771% 1.949 27.449]
and
[32.102% 49.603 -20.341].
The midpoint of linearly interpolating these would be
[29.4365% 25.776 3.554]
which, with an alpha value of 0.5,
is
lab(58.873% 51.552 7.108) / 0.5)
when premultiplication is undone.
To interpolate, in the chroma-preserving LCH color space, the same two colors
rgb(76% 62% 03% / 0.4)
and
color(display-p3 0.84 0.19 0.72 / 0.6)
they are first converted to LCH
lch(66.93% 68.79 85.94 / 0.4)
lch(53.5% 89.35 337.7 / 0.6)
then the L and C coordinates (but not H) are premultiplied before interpolation
[26.771% 27.516 85.94]
and
[32.102% 53.61 337.7].
The midpoint of linearly interpolating these,
along the
shorter
hue arc (the default) would be
[29.4365% 40.563 31.82]
which, with an alpha value of 0.5,
is
lch(58.873% 81.126 31.82) / 0.5)
when premultiplication is undone.
There is sample JavaScript code
for alpha premultiplication and un-premultiplication,
for both polar and rectangular color spaces,
in
§ 19 Sample code for Color Conversions
13.4.
Hue Interpolation
For color functions with a hue angle (LCH, HSL, HWB etc), there are multiple ways to interpolate.
As arcs greater than 360° are rarely desirable,
hue angles are fixed up prior to interpolation
so that per-component interpolation is done over less than 360°, often less than 180°.
Host syntax can specify any of the following algorithms for hue interpolation
(angles in the following are in degrees, but the logic is the same regardless of how they are specified).
Specifying a hue interpolation strategy is already part of the

syntax
via the

token.
Unless otherwise specified, if no specific hue interpolation algorithm is selected by the host syntax, the default is
shorter
Tests
color-mix-percents-01.html
(live test)
(source)
color-mix-percents-02.html
(live test)
(source)
Note:
As a reminder,
if the interpolating colors were not already in the specified interpolation color space,
then converting them will turn any
powerless components
into
missing components
13.4.1.
shorter
Hue angles are interpolated to take the
shorter
of the two arcs
between the starting and ending hues.
For example, the midpoint when interpolating in OkLCh from a red
oklch(0.6 0.24 30) to a yellow
oklch(0.8 0.15 90)
would be at a hue angle of 30 + (90 - 30) * 0.5 = 60 degrees,
along the shorter arc between the two colors,
giving a deep orange
oklch(0.7 0.195 60)
Angles are adjusted so that
θ₂ - θ₁
∈ [-180, 180]. In pseudo-Javascript:
if
θ₂ - θ₁ >
180
θ₁ +=
360
else if
θ₂ - θ₁ <
-180
θ₂ +=
360
13.4.2.
longer
Hue angles are interpolated to take the
longer
of the two arcs
between the starting and ending hues.
For example, the midpoint when interpolating in OkLCh from a red
oklch(0.6 0.24 30) to a yellow
oklch(0.8 0.15 90)
would be at a hue angle of (30 + 360 + 90) * 0.5 = 240 degrees,
along the longer arc between the two colors,
giving a sky blue
oklch(0.7 0.195 240)
Angles are adjusted so that
θ₂ - θ₁
∈ {(-360, -180], [180, 360)}. In pseudo-Javascript:
if
< θ₂ - θ₁ <
180
θ₁ +=
360
else if
-180
< θ₂ - θ₁ <=
θ₂ +=
360
13.4.3.
increasing
Hue angles are interpolated so that,
as they progress from the first color to the second,
the angle is always
increasing
If the angle increases to 360 it is reset to zero,
and then continues increasing.
Depending on the difference between the two angles,
this will either look the same as
shorter
or as
longer.
However, if one of the hue angles is being animated,
and the hue angle difference passes through 180 degrees,
the interpolation will not flip to the other arc.
For example, the midpoint when interpolating in OkLCh from a deep brown
oklch(0.5 0.1 30) to a turquoise
oklch(0.7 0.1 190)
would be at a hue angle of (30 + 190) * 0.5 = 110 degrees,
giving a khaki
oklch(0.6 0.1 110).
However, if the hue of the second color is animated to
oklch(0.7 0.1 230),
the midpoint of the interpolation will be (30 + 230) * 0.5 = 130 degrees,
continuing in the same increasing direction,
giving another green
oklch(0.6 0.1 130)
rather than flipping to the opponent color part-way through the animation.
Angles are adjusted so that
θ₂ - θ₁
∈ [0, 360). In pseudo-Javascript:
if
θ₂ < θ₁
θ₂ +=
360
13.4.4.
decreasing
Hue angles are interpolated so that,
as they progress from the first color to the second,
the angle is always
decreasing
If the angle decreases to 0 it is reset to 360,
and then continues decreasing.
Depending on the difference between the two angles,
this will either look the same as
shorter
or as
longer.
However, if one of the hue angles is being animated,
and the hue angle difference passes through 180 degrees,
the interpolation will not flip to the other arc.
For example, the midpoint when interpolating in OkLCh from a deep brown
oklch(0.5 0.1 30) to a turquoise
oklch(0.7 0.1 190)
would be at a hue angle of (30 + 360 + 190) * 0.5 = 290 degrees,
giving a purple
oklch(0.6 0.1 290).
However, if the hue of the second color is animated to
oklch(0.7 0.1 230),
the midpoint of the interpolation will be (30 + 360 + 230) * 0.5 = 310 degrees,
continuing in the same decreasing direction,
giving another purple
oklch(0.6 0.1 310)
rather than flipping to the opponent color part-way through the animation.
Angles are adjusted so that
θ₂ - θ₁
∈ (-360, 0]. In pseudo-Javascript:
if
θ₁ < θ₂
θ₁ +=
360
14.
Gamut Mapping
14.1.
An Introduction to Gamut Mapping
Note:
This section provides important context for the specific requirements described elsewhere in the document.
This section is non-normative
Tests
This section is not normative, it does not need tests.
When a color in an origin color space
is converted to another, destination color space
which has a smaller gamut,
some colors will be outside the destination gamut.
For intermediate color calculations,
these out of gamut values are preserved.
However, if the destination is the display device
(a screen, or a printer)
then out of gamut values must be converted to
an in-gamut color.
Gamut mapping is the process of finding an in-gamut color
with the least objectionable change in visual appearance.
Some out of gamut colors correspond to real-world colors
(they could be physically reproduced)
while others are imaginary colors
(they lie outside the spectral locus
and thus would need more than 100% of a single wavelength)
and could never be physically realized.
Those colors tend to come from calculations
such as "make this color 100x as saturated".
For example, while theoretically unbounded,
the CIE Lab
(a,b)
plane is often implemented
such that
and
are constrained
to the range ±127.
Three of those four corners are outside the spectral locus
and thus correspond to imaginary colors.
The four corners of the ±127 (a,b) plane,
on a UCS chromaticity diagram.
The outer shape is the spectral locus.
The larger triangle is the rec2020 gamut,
while the smaller is sRGB.
14.1.1.
Clipping
The simplest and least acceptable method
is simply to clip the component values
to the displayable range.
Since the motivation for this method is speed,
clipping is commonly done on gamma-encoded values
rather than converting them to linear-light.
This changes the proportions of
the three primary colors (for an RGB display),
resulting in a hue shift.
For example,
consider the color
color
srgb-linear
0.5
Because this is a linear-light color space,
we can compare the intensities of the three components
and see that
the amount of blue light is three times the amount of green,
while the amount of red light is half that of green.
There is six times as much blue primary as red.
In OkLCh, this color has a hue angle of 265.1°
If we now clip this color
to bring it into gamut for sRGB,
we get
color
srgb-linear
0.5
The amount of blue light is the same as green.
In OkLCh, this color has a hue angle of 196.1°,
a substantial change of 69°.
When colors are not too far out of gamut,
clipping can give acceptable results.
This is particularly true for darker (or negative)
component values.
For example,
consider the color
color
rec2020
0.54
0.9
which is
oklch
80.72
0.3296
141.6
Converting to p3 colorspace, the negative blue value shows
the color is out of gamut:
color
display-p3
0.3265
0.9165
-0.1262
which in linear-light is
color
display-p3-linear
0.0871
0.8205
-0.0146
Clipping the gamma-encoded p3 color to the p3 gamut gives
color
display-p3
0.3265
0.9165
which in linear-light is
color
display-p3-linear
0.0871
0.8205
which, for comparison, is
oklch
80.79
0.3221
142.3
This is a good result;
the hue angle and lightness have barely changed
but the chroma is somewhat reduced, as expected.
In terms of percentages of linear-light red green and blue,
the red and green are identical
while the blue is -1.46% higher.
For example,
consider the color
color
prophoto-rgb
0.2
1.0
0.1
which is
oklch
85.07
0.4873
151.4
Converting to p3 colorspace, the color is significantly
out of gamut:
color
display-p3
-0.5782
1.067
-0.2363
which in linear-light is
color
display-p3-linear
-0.2937
1.158
-0.0456
Clipping the gamma-encoded p3 color to the p3 gamut gives
color
display-p3
which in linear-light is, again,
color
display-p3-linear
(component values of exactly 0 or 1 are unaffected by gamma encoding)
which, for comparison, is
oklch
84.88
0.3685
145.6
A less good but still visually acceptable result.
Here the hue is more affected, a 5.8° change.
In terms of percentages of linear-light red green and blue,
red is 57% higher, green is 6.7% lower and blue is 23% higher.
14.1.2.
Closest Color (MINDE)
A better method is to map colors,
in a perceptually uniform color space,
by finding the closest in-gamut color
(so-called minimum ΔE or
MINDE
).
Clearly, the success of this technique
depends on
the degree of uniformity of the gamut mapping color space
and the predictive accuracy of the deltaE function used.
However, when doing gamut mapping
changes in Hue are
particularly
objectionable;
changes in Chroma are more tolerable,
and
small changes in Lightness can also be acceptable
especially if the alternative is a larger Chroma reduction.
MINDE weights changes in each dimension equally,
and thus gives suboptimal results.
14.1.3.
Chroma Reduction
To implement MINDE,
colors are mapped in a perceptually uniform,
polar
color space
by holding the hue constant,
and reducing the chroma until the color falls in gamut.
This could be done
algorithmically
by finding the geometric intersection
of a constant-lightness, constant-hue ray
with the gamut boundary;
or
iteratively
reducing the chroma until it falls in gamut.
Note:
For performance reasons,
iteration is typically performed by binary search.
In this example, Display P3 primary yellow
color
display-p3
is being mapped to an sRGB display.
The gamut mapping color space is OkLCh.
color
display-p3
is
color
srgb
-0.3463
which is
color
oklch
0.96476
0.24503
110.23
By progressively reducing the chroma component
until the resulting color falls inside the sRGB gamut
(has no components negative, or greater than one)
a gamut mapped color is obtained.
color
oklch
0.96476
0.21094
110.23
which is
color
srgb
0.99116
0.99733
0.00001
A constant-hue slice of OkLCh color space.
The vertical axis represents lightness,
the horizontal axis is chroma.
The color to be mapped,
shown as a yellow circle,
has the chroma reduced
while keeping hue and lightness constant.
The color therefore moves along the maroon line in the diagram,
towards the neutral axis on the left.
The gamut boundary of sRGB
is shown in green.
14.1.4.
Excessive Chroma Reduction
Also, this simple MINDE approach will give sub-optimal results
for certain colors, principally very light colors
like yellow and cyan,
if the upper edge of the gamut boundary is shallow,
or even slightly concave.
The line of constant lightness can skim just above the gamut boundary,
resulting in an excessively low chroma in those cases.
The choice of color space will affect the acceptability of the gamut mapped colors.
In this example, Display P3 primary yellow (
color
display-p3
has the chroma progressively reduced in CIE LCH color space.
In the upper part of this diagram,
colors which are inside the gamut of sRGB are displayed as-is.
Colors inside the gamut of Display P3 (but outside sRGB) are in salmon.
Colors outside the gamut of Display P3 are in red.
The lower part of the diagram shows the linear-light intensities of
the Display P3 red, green and blue components.
It can be seen that reduction in CIE LCH chroma makes the red
intensity curve up, out of Display P3 gamut;
by the time it falls again the chroma is very low.
Simple gamut mapping in CIE LCH would give unsatisfactory results.
In this example, Display P3 primary yellow (
color
display-p3
has the chroma progressively reduced, but this time in OkLCh color space.
In the upper part of this diagram,
colors which are inside the gamut of sRGB are displayed as-is.
Colors inside the gamut of Display P3 (but outside sRGB) are in salmon.
Colors outside the gamut of Display P3 are in red.
The lower part of the diagram shows the linear-light intensities of
the Display P3 red, green and blue components.
It can be seen that reduction in OkLCh chroma is better behaved.
Colors do not go outside the Display P3 gamut, and the resulting
gamut-mapped yellow has good chroma.
Simple gamut mapping in OK LCH would give acceptable results.
14.1.5.
Chroma Reduction with Local Clipping
The simple chroma-reduction algorithm can be improved:
at each step,
the color difference is computed between the current mapped color
and a clipped version of that color.
If the current color is outside the gamut boundary,
but the color difference between it and the clipped version
is below the threshold for a
just noticeable difference
(JND),
the clipped version of the color is returned as the mapped result.
Effectively, this is doing a MINDE mapping at each stage,
but constrained so the hue and lightness changes
are very small,
and thus are not noticeable.
In this example, Display P3 primary yellow (
color
display-p3
has the chroma progressively reduced in CIE LCH color space,
with the local clip modification.
In the upper part of this diagram,
colors which are inside the gamut of sRGB are displayed as-is.
Colors inside the gamut of Display P3 (but outside sRGB) are in salmon.
Colors outside the gamut of Display P3 are in red.
The lower part of the diagram shows the linear-light intensities of
the Display P3 red, green and blue components.
It can be seen that reduction in CIE LCH chroma still makes the red
intensity curve up, out of Display P3 gamut;
but less than before and the sRGB boundary is found much more quickly.
Gamut mapping in CIE LCH with local clip would give acceptable results.
In this example, Display P3 primary yellow (
color
display-p3
has the chroma progressively reduced, but this time in OkLCh color space
and with the local clip modification.
In the upper part of this diagram,
colors which are inside the gamut of sRGB are displayed as-is.
Colors inside the gamut of Display P3 (but outside sRGB) are in salmon.
Colors outside the gamut of Display P3 are in red.
The lower part of the diagram shows the linear-light intensities of
the Display P3 red, green and blue components.
It can be seen that reduction in OkLCh chroma,
which was already good,
is further improved by the local clip modification.
Simple gamut mapping in CIE LCH with local clip would give excellent results.
14.1.6.
Deviations from Perceptual Uniformity: Hue Curvature
Performing gamut mapping in the CIE LCH color space
even with the deltaE2000 distance metric,
is known to give suboptimal results
with significant hue shifts,
for colors in the hue range
270° to 330°.
A constant-hue slice of CIE LCH color space,
at a hue angle of 301.37°
corresponding to sRGB primary blue.
The vertical axis is Lightness, the horizontal axis is Chroma.
Between chroma of 25 and 75, the hue is visibly purple,
becoming more blue between 100 and 131.
The same phenomenon continues past 131,
but cannot be shown on an sRGB display.
Using OkLCh color space
and the deltaEOK distance metric
avoids this issue
at all hue angles.
A constant-hue slice of OkLCh color space,
at a hue angle of 264.06°
corresponding to sRGB primary blue.
The vertical axis is Lightness, the horizontal axis is Chroma.
The hue is visibly the same at all values of chroma,
up to 0.315 (the sRGB limit at this hue).
It continues to be constant beyond this point,
although that cannot be shown on an sRGB diagram.
14.2.
CSS Gamut Mapping to an RGB Destination
Tests
Actual values of color are not exposed to script, making this hard to test in an automated manner.
The three
CSS gamut mapping algorithms
apply to individual,
Standard Dynamic Range (SDR) CSS colors
which are out of gamut
of an RGB display
and thus require to be
css gamut mapped
Implementations my choose any of the three algorithms
based on their quality and runtime efficiency tradeoffs,
and must use their chosen algorithm
wherever CSS mandates that gamut mapping be performed.
Binary Search Gamut Mapping with Local MINDE
EdgeSeeker Gamut Mapping
Ray Trace Gamut Mapping
They all implement a relative colorimetric intent,
thus colors inside the destination gamut are unchanged.
Note:
other situations,
in particular mapping to printer gamuts
where the maximum black level is significantly above zero,
will require different algorithms
which align the respective black and white points,
which will result in lightness changes
for very light and very dark colors
as chroma is reduced..
Note:
these algorithms are for individual, distinct colors;
for color images,
where relationships between neighboring pixels are important
and the aim is to preserve detail and texture,
a perceptual rendering intent is more appropriate
and in that case,
colors inside the destination gamut
could be changed.
All three CSS gamut mapping algorithms aim at
constant-lightness, constant-hue chroma reduction
in the
OkLCh color space
For colors which are out of range on the Lightness axis,
white is returned in the destination color space
if the Lightness is greater than or equal to 1.0,
while black is returned in the destination color space
if the Lightness is less than or equal to 0.0.
14.2.1.
Binary Search Gamut Mapping with Local MINDE
For this binary search algorithm,
the color difference formula used is
deltaEOK
The
local-MINDE
improvement is used.
At each step in the search,
the deltaEOK is computed between the current mapped color
and a clipped version of that color.
If the current color is
outside
the gamut boundary,
but the deltaEOK between it and the clipped version
is below a threshold for a
just noticeable difference
(JND),
the clipped version of the color is returned as the mapped result.
This gives good results with non-notivceable hue shifts,
and avoids excessive chroma reduction near concave gamut surfaces,
but can be computationally intensive.
For the OkLCh color space,
one JND is is an OkLCh difference of 0.02.
Note:
In CIE Lab color space,
where the range of the Lightness component is 0 to 100,
using deltaE2000,
one JND is 2.
Because the range of Lightness in Oklab and OkLCh
is 0 to 1,
using deltaEOK,
one JND is 100 times smaller.
Note:
for the purposes of experimentation,
and comparing implementations,
implementations of the Binary Search with Local MINDE algorithm are available
in the Coloriade library (in Python)
[Coloraide-MINDE]
and the color.js library (in JavaScript)
[colorjs-MINDE]
14.2.2.
Sample Pseudocode for the Binary Search Gamut Mapping with Local MINDE
To
Binary Search Gamut Map with Local MINDE
a color
origin
in color space
origin color space
to be in gamut of a destination color space
destination
if
destination
has no gamut limits (XYZ-D65, XYZ-D50, Lab, LCH, Oklab, OkLCh) convert
origin
to
destination
and return it as the gamut mapped color
let
origin_OkLCh
be
origin
converted
from
origin color space
to the OkLCh color space
if the Lightness of
origin_OkLCh
is greater than or equal to 100%,
convert `oklab(1 0 0 / origin.alpha)` to
destination
and return it as the gamut mapped color
if the Lightness of
origin_OkLCh
is less than than or equal to 0%,
convert `oklab(0 0 0 / origin.alpha)` to
destination
and return it as the gamut mapped color
let inGamut(
color
) be a function which returns true if, when passed a color,
that color is inside the gamut of
destination
For HSL and HWB, it returns true if the color is inside the gamut of sRGB.
if inGamut(
origin_OkLCh
) is true, convert
origin_OkLCh
to
destination
and return it as the gamut mapped color
otherwise, let delta(
one
two
) be a function which returns the deltaEOK of color
one
compared to color
two
let
JND
be 0.02
let
epsilon
be 0.0001
let clip(
color
) be a function which converts
color
to
destination
clamps each component to the bounds of the reference range for that component
and returns the result
set
current
to
origin_OkLCh
set
clipped
to clip(
current
set
to delta(
clipped
current
if
JND
return
clipped
as the gamut mapped color
set
min
to zero
set
max
to the OkLCh chroma of
origin_OkLCh
let
min_inGamut
be a boolean that represents when
min
is still in gamut, and set it to true
while (
max
min
is greater than
epsilon
) repeat the following steps
set
chroma
to (
min
max
) /2
set the chroma component of
current
to
chroma
if
min_inGamut
is true and also if inGamut(
current
) is true, set
min
to
chroma
and continue to repeat these steps
otherwise, carry out these steps:
set
clipped
to clip(
current
set
to delta(
clipped
current
if
JND
if (
JND
epsilon
) return
clipped
as the gamut mapped color
otherwise,
set
min_inGamut
to false
set
min
to
chroma
otherwise, set
max
to
chroma
and continue to repeat these steps
return
clipped
as the gamut mapped color
14.2.3.
The EdgeSeeker Gamut Mapping
The EdgeSeeker algorithm is a geometric and lookup-table-based approach,
originally developed by Alexey Ardov
for the color.js library
[colorjs-EdgeSeeker]
For any given hue, the gamut boundary slice
is represented as a curved top section
and a linear bottom section,
joning at the highest chroma point for that hue.
To initialize this algorithm,
for a given target RGB space,
a lookup table (LUT) is constructed
containing the highest-chroma Oklch color on each hue slice.
Linear interpolation between closest LUT values
is used to then estimate the highest-chroma color
for the exact hue of each color to be gamut mapped.
Intersection of the constant-lightness ray
with the gamut boundary is then calculated,
which is fast for the lower (linear) part of the boundary
and still fairly fast for the upper (curved) portion.
This gives good results, at the expense of memory for the LUT.
14.2.4.
Sample Pseudocode for the EdgeSeeker Gamut Mapping
add pseudocode for EdgeSeeker GMA
14.2.5.
The Ray Trace Gamut Mapping
The Ray Trace algorithm is a geometric approach
to RGB gamut mapping,
for fast chroma reduction with constant lightness.
It was originally developed by Isaac Muse
for the Coloraide Python Library
[Coloraide-Ray-Trace]
The color to be mapped is first converted to Oklch,
and then the achromatic version of that color is generated,
which will be the neutral axis anchor.
These two colors are then converted to
the linear-light version of the target RGB space.
Because the gamut boundary is now an axis-aligned cube,
finding the intersection is faster.
A ray is cast from the inside of the RGB cube,
from the anchor point to the current color.
The intersection along this path
with the RGB gamut surface is then found;
this is the first approximation to the gamut mapped color.
As RGB spaces are not perceptually uniform,
a constant hue, constant lightness ray
is actually a curved path in RGB space.
The first approximation is converted back to Oklch
to correct the color in the perceptual color space
by projecting the point back onto the chroma reduction path,
correcting the color’s hue and lightness.
The corrected color becomes the new current color
and should be a much closer color on the reduced chroma line.
This process is repeated (a maximum of three more times),
each time finding a better, closer color on the path.
Finally, simple clipping is used to account for floating point math errors.
The results are comparable to binary search with local MINDE
using a low JND,
but resolves much faster
and within more predictable, consistent time.
Ray Trace gamut mpping to the sRGB gamut,
showing the curved path of chroma reduction
as approximated over a maximum
of four iterations.
Image copyright Isaac Muse.
Note:
for the purposes of experimentation,
and comparing implementations,
implementations of the Ray Trace are available
in the Coloriade library (in Python)
[Coloraide-Ray-Trace]
and the color.js library (in JavaScript)
[colorjs-RayTrace]
14.2.6.
Sample Pseudocode for the Ray Trace Gamut Mapping
To
Ray Trace Gamut Map
a color
origin
in color space
origin color space
to be in gamut of an RGB destination color space
destination
if
destination
has no gamut limits (XYZ-D65, XYZ-D50, Lab, LCH, Oklab, OkLCh) convert
origin
to
destination
and return it as the gamut mapped color
let
origin_OkLCh
be
origin
converted from
origin color space
to the OkLCh color space
if the Lightness of
origin_OkLCh
is greater than or equal to 100%,
convert `oklab(1 0 0 / origin.alpha)` to
destination
and return it as the gamut mapped color
if the Lightness of
origin_OkLCh
is less than than or equal to 0%,
convert `oklab(0 0 0 / origin.alpha)` to
destination
and return it as the gamut mapped color
let
l_origin
be the OkLCh lightness component of
origin_OkLCh
let
h_origin
be the OkLCh hue component of
origin_OkLCh
let
anchor
be an achromatic OkLCh color formed with
l_origin
as lightness, 0 as chroma and
h_origin
as hue,
converted to the
linear-light
form of
destination
let
origin_rgb
be
origin_OkLCh
converted to the
linear-light
form of
destination
if
origin_rgb
is not in gamut
let
low
be 0.0 + 1E-6
let
high
be 1.0 - 1E-6
let
last
be
origin_rgb
for (i=0; i<4; i++)
if (i > 0)
let
current_OkLCh
be
origin_rgb
converted to OkLCh
let the lightness of
current_OkLCh
be
l_origin
let the hue of
current_OkLCh
be
h_origin
let
origin_rgb
be
current_OkLCh
converted to the
linear-light
form of
destination
Cast a ray
from
start
anchor
to
end
origin_rgb
and let
intersection
be
the intersection of this ray
with the gamut boundary
if an intersection was not found,
let
origin_rgb
be
last
and exit the loop
if (i >0) AND (each component of
origin_rgb
is between
low
and
high
) then
let
anchor
be
origin_rgb
let
origin_rgb
be
intersection
let
last
be
intersection
let clip(
color
) be a function which converts
color
to
destination
clamps each component to the bounds of the reference range for that component
and returns the result
set
clipped
to clip(
current
return
clipped
as the gamut mapped color
To
cast a ray
through a linear-light RGB space
from
start
to
end
(in gamut mapping,
start
is an anchor within the RGB gamut
and
end
is the gamut mapped color, on the cubical gamut surface):
let
bmin
and
bmax
be 3-element arrays
with the gamut’s lower and upper bounds, respectively
let
tfar
be infinity (or some very large number)
let
tnear
be -infinity (or some very large, negative number)
let
direction
be a 3-element array
for (i = 0; i < 3; i++):
let
be
start
[i]
let
be
end
[i]
let
be
let
direction
[i]
be
if abs(
) < 1E-12
let
inv_d
be 1 /
let
t1
be (
bmin
[i]
) *
inv_d
let
t2
be (
bmax
[i]
) *
inv_d
let
tnear
be max(min(
t1
t2
),
tnear
let
tfar
be min(max(
t1
t2
),
tfar
else if (
bmin
[i]
or
bmax
[i]
return INTERSECTION NOT FOUND
if (
tnear
tfar
or
tfar
< 0)
return INTERSECTION NOT FOUND
if
tnear
< 0
let
tnear
be
tfar
if
tnear
is infinite (or matches the initial very large value)
return INTERSECTION NOT FOUND
for (i =0; i < 3; i++):
let
result
[i]
be
start
[i]
direction
[i]
tnear
return
result
14.2.6.1.
Footnotes for Ray Trace algorithm
It is assumed the minimum value is 0
and that all channels have the same minimum.
The value should be small relative to the unit type.
64 bit could easily be as small as 1e-14, but 1e-6 is fine in practice.
1.0 represents the maximum in-gamut channel value,
and it is assumed all channels have the same maximum.
This places the
current
color back on the chroma reduction curve,
if it has deviated.
This means
origin_rgb
is below the gamut surface,
so we use it as an anchor closer to the gamut surface.
This is provided for catastrophic failures
where a specific, perceptual mapping space
completely breaks down
due to ridiculously wide colors (outside the visible spectrum).
It is expected that non-imaginary colors in CSS should never trigger this.
For typical RGB spaces
where the gamut bounds are 0 and 1 for each component
this simplifies to a single constant
rather than a 3-element array.
favoring the first intersection in the direction
start
->
end
15.
Resolving

Values
Unless otherwise specified for a particular property,
specified
colors are resolved to
computed
colors
and then further to
used
colors
as described below.
The
resolved value
of a

is its
used value
Tests
color-computed-hex-color.html
(live test)
(source)
color-computed-named-color.html
(live test)
(source)
color-invalid-hex-color.html
(live test)
(source)
color-invalid-named-color.html
(live test)
(source)
system-color-compute.html
(live test)
(source)
15.1.
Resolving sRGB values
This applies to:
hex colors
rgb()
and
rgba()
values
hsl()
and
hsla()
values
hwb()
values
named colors
system colors
deprecated-colors
It does
not
apply to:
color()
values using the
srgb
or
srgb-linear
color space
s.
If the sRGB color was explicitly specified by the author as a
named color
or as a
system color
the
declared value
is that named or system color, converted to
ASCII lowercase
The computed and used value
is the corresponding sRGB color,
paired with the specified alpha component
(after clamping to [0, 1])
and defaulting to opaque if unspecified).
The author-provided mixed-case form below has a declared value in all lowercase.
pUrPlE
purple
Otherwise, the declared, computed and used value
is the corresponding sRGB color,
paired with the specified alpha component
(after clamping to [0, 1])
and defaulting to opaque if unspecified).
For historical reasons, when
calc()
in sRGB colors
resolves to a single value,
the declared value serialises without the "calc(" ")" wrapper.
For example, if a color is given as
rgb(calc(64 * 2) 127 255)
the declared value will be
rgb(128 127 255)
and not
rgb(calc(128) 127 255).
For example, if a color is given as
hsl(38.82 calc(2 * 50%) 50%)
the declared value will be
rgb(255 165.2 0)
because the
calc()
is lost
during HSL to RGB conversion.
Also for historical reasons,
when calc() is simplified down to a single value,
the color values are clamped to [0.0, 255.0].
For example, if a color is given as
rgb(calc(100 * 4) 127 calc(20 - 35))
the declared value will be
rgb(255 127 0)
and not
rgb(calc(400) 127 calc(-15)).
This clamping also takes care of values such as
Infinity
-Infinity
, and
NaN
which will clamp at 255, 0 and 0 respectively.
For example, the computed value of
hsl
38.824
100
50
is
rgb
255
165
Tests
color-computed-hsl.html
(live test)
(source)
color-computed-hwb.html
(live test)
(source)
color-computed-rgb.html
(live test)
(source)
15.2.
Resolving Lab and LCH values
This applies to
lab()
and
lch()
values.
The declared, computed and used value
is the corresponding CIE Lab or LCH color
(after clamping of L, C and H)
paired with the specified alpha component
(as a

, not a

and defaulting to opaque if unspecified).
For example, the computed value of
lch
52.2345
72.2
56.2
is
lch
52.2345
72.2
56.2
Although the values of a, b and C
are theoretically unbounded,
there may be an
implementation-defined limit for values approaching infinity
Tests
color-computed-lab.html
(live test)
(source)
15.3.
Resolving Oklab and OkLCh values
This applies to
oklab()
and
oklch()
values.
The declared, computed and used value
is the corresponding Oklab or OkLCh color
(after clamping of L, C and H)
paired with the specified alpha component
(as a

, not a

and defaulting to opaque if unspecified).
For example, the computed value of
oklch
42.1
0.192
328.6
is
oklch
42.1
0.192
328.6
Although the values of a, b and C
are theoretically unbounded,
there may be an
implementation-defined limit for values approaching infinity
Tests
color-computed-lab.html
(live test)
(source)
15.4.
Resolving values of the
color()
function
The declared, computed and used value
is the color in the specified
color space
paired with the specified alpha component
(as a

, not a

and defaulting to opaque if unspecified).
For example, the computed value of
color
display-p3
0.823
0.6554
0.2537
is
color
display-p3
0.823
0.6554
0.2537
For colors specified in the
xyz
color space
which is an alias of the
xyz-d65
color space
the computed and used value
is in the
xyz-d65
color space
For example, the computed value of
color
xyz
0.472
0.372
0.131
is
color
xyz-d65
0.472
0.372
0.131
Although the values of r, g, b, x, y and z
are theoretically unbounded,
there may be an
implementation-defined limit for values approaching infinity
Tests
color-computed-color-function.html
(live test)
(source)
15.5.
Resolving other colors
This applies to
system colors
(including the

s),
transparent
and
currentcolor
The declared value for each

keyword
and

keyword
is itself.
The computed value
is the corresponding color in its color space.
However, such colors must not be altered by
forced colors mode
For example, in this html:
button
style
"color:
ButtonText; background:
ButtonFace"
>button
The declared value of the color property is "ButtonText"
while the computed value could be, for example,
rgb(0, 0, 0).
The declared value of
transparent
is "transparent"
while the computed and used value is
transparent black
The
currentcolor
keyword computes to itself.
In the
color
property,
the used value of
currentcolor
is the
resolved
inherited value
In any other property,
its used value is the used value of the
color
property on the same element.
Note:
This means that if the
currentcolor
value is inherited,
it’s inherited as a keyword,
not as the value of the
color
property,
so descendants will use their own
color
property to resolve it.
For example, given this html:
div
Assume this example text is long enough
to wrap on multiple lines.
div
and this css:
div
color
forestgreen
text-shadow
currentColor
color
mediumseagreen
p::firstline
color
yellowgreen
The used value of the inherited property text-shadow
on the first line fragment would be yellowgreen.
Tests
currentcolor-001.html
(live test)
(source)
currentcolor-002.html
(live test)
(source)
currentcolor-003.html
(live test)
(source)
currentcolor-005.html
(live test)
(source)
system-color-compute.html
(live test)
(source)
16.
Serializing

Values
This section updates and replaces that part of CSS Object Model, section
Serializing CSS Values
, which relates to serializing

values.
In this section, the strings used in the specification and the corresponding characters are as follows.
String
Character(s)
" "
U+0020 SPACE
"#"
U+0023 NUMBER SIGN
","
U+002C COMMA
"-"
U+002D HYPHEN-MINUS
"."
U+002E FULL STOP
"/"
U+002F SOLIDUS
"none"
U+006E LATIN SMALL LETTER N
U+006F LATIN SMALL LETTER O
U+006E LATIN SMALL LETTER N
U+0065 LATIN SMALL LETTER E
The string "." shall be used as a decimal separator,
regardless of locale,
and there shall be no thousands separator.
For syntactic forms which support
missing color components
the value
none
(equivalently NONE, nOnE, etc),
shall be serialized in all-lowercase
as the string "none".
16.1.
Serializing alpha values
This applies to any

value which can take an optional alpha value.
It does not apply to the
opacity
property.
If, after clamping to the range [0, 1] the alpha is 1,
it is omitted from the serialization;
an implicit value of 1 (fully opaque) is the default.
If the alpha is any other value than 1,
it is explicitly included in the serialization as described below.
If the value is internally represented as an integer
between 0 and 255 inclusive (i.e. 8-bit unsigned integer),
follow these steps:
Let
alpha
be the given integer.
If there exists an integer between 0 and 100 inclusive that,
when multiplied with 2.55 and rounded to the closest integer
(rounding up if two values are equally close), equals
alpha
let
rounded
be that integer divided by 100.
Otherwise, let
rounded
be
alpha
divided by 0.255 and rounded to the closest integer
(rounding up if two values are equally close),
divided by 1000.
Return the result of serializing
rounded
as a

Otherwise, return the result of serializing the given value
(as a

, not a

).
For example,
if the alpha is stored as the 8-bit unsigned integer 237,
the integer 93 satisfies the criterion
because Math.round(93 * 2.55) is 237,
and so the alpha is serialized as "0.93".
However,
if the alpha is stored as the 8-bit unsigned integer 236,
there is no such integer
(92 maps to 235 while 94 maps to 240),
and so since 236 ÷ 0.255 = 925.490196078
the alpha is serialized as "0.92549"
(no more than 6 figures, trailing zeroes omitted).
The

value is expressed in base ten,
with the "." character as decimal separator.
The leading zero must not be omitted.
Trailing zeroes must be omitted.
For example, an alpha value of 70%
will be serialized as the string
"0.7"
which has a leading zero before the decimal separator,
"." as decimal separator
(even if the current locale would use some other character,
such as ","),
and all digits after the "7" would be "0" and are omitted.
The precision with which alpha values are retained,
and thus the number of decimal places in the serialized value,
is not defined in this specification,
but must at least be sufficient
to round-trip integer percentage values.
Thus, the serialized value must contain
at least two decimal places
(unless trailing zeroes have been removed).
Values must be
rounded towards +∞
, not truncated.
For example, an alpha value of 12.3456789%
could be serialized as the strings
"0.12" or "0.123" or "0.1234" or "0.12346"
(rounding the value of 5
towards +∞
because the following digit is 6)
or any longer, rounded serialization of the same form.
Because

s which were specified outside the valid range
are clamped at parse time, the declared value will be clamped.
However, per
CSS Values 4
§ 10.12 Range Checking

specified using calc() are not clamped when the specified form is serialized;
but the computed values are clamped.
For example an alpha value which was specified directly as 120%
would be serialized as the string "1".
However, if it was specified as calc(2*60%)
the declared value would be serialized as the string "calc(1.2)".
16.2.
Serializing sRGB values
The serialized form of the following sRGB values:
hex colors
rgb()
and
rgba()
values
hsl()
and
hsla()
values
hwb()
values
named colors
system colors
deprecated-colors
transparent
is derived from the
declared value
When serializing the value of a property
which was set by the author to a CSS
named color
system color
deprecated-color
or
transparent
therefore, for the
declared value
the
ASCII lowercase
keyword value is retained.
For the computed and used value,
the corresponding sRGB value is used.
Tests
system-color-compute.html
(live test)
(source)
Thus, the serialized declared value of
transparent
is the string "transparent",
while the serialized computed value of
transparent
is the string "rgba(0, 0, 0, 0)".
For all other sRGB values,
the declared, computed and used value
is the corresponding sRGB value.
During serialization,
any
missing
values
are converted to 0.
16.2.1.
HTML-compatible serialization of sRGB values
If the following conditions are all true:
The color space is sRGB
The alpha is 1
The RGB component values are internally represented as integers between 0 and 255 inclusive (i.e. 8-bit unsigned integer)
HTML-compatible serialization is requested
Then corresponding sRGB values are serialized in 6-digit
hex color notation
as follows:
A seven-character string consisting of the character "#", followed immediately by the two-digit hexadecimal representations of the red component, the green component, and the blue component, in that order, using
ASCII lower hex digits
. No spaces are permitted.
For example, fill style is set to
magenta:
context
fillStyle
"rgb(255, 0, 255)"
console
log
context
fillStyle
);
// "#ff00ff"
The color space is sRGB, the representation is 8 bits per component,
the data format does not produce
none
values nor does it support extended range values,
and the alpha is 1.
The HTML-compatible serialization is the string "#ff00ff" (not "#FF00FF").
Otherwise, for sRGB the
CSS serialization of sRGB values is used
and for other color spaces, the relevant
serialization
of the

value.
For example, fill style is set to
a dark brown, in CIE Lab:
context
fillStyle
"lab(29% 39 20)"
console
log
context
fillStyle
);
// "lab(29 39 20)"
The CSS serialization is the string "lab(29 39 20)".
For example, fill style is set to
semi-transparent magenta:
context
fillStyle
"#ff00ffed"
console
log
context
fillStyle
);
// "rgba(255, 0, 255, 0.93)"
The alpha is not 1, so the CSS serialization is the string
"rgba(255, 0, 255, 0.93)".
16.2.2.
CSS serialization of sRGB values
Corresponding sRGB values use either the
rgb()
or
rgba()
form
(depending on whether the (clamped) alpha is exactly 1, or not),
with all
ASCII lowercase
letters for the function name.
For compatibility, the sRGB component values
are serialized in

form, not

Also for compatibility,
the component values are serialized in base 10,
with a range of [0-255], regardless of
the bit depth with which they are stored.
As noted earlier
unitary alpha values are not explicitly serialized.
Also, for compatibility, if the alpha is exactly 1,
the
rgb()
form is used,
with an implicit alpha;
otherwise, the
rgba()
form is used,
with an explicit alpha value.
For compatibility,
the legacy form with comma separators is used;
exactly one ASCII space follows each comma.
This includes the comma (not slash) used
to separate the blue component of
rgba()
from the alpha value.
For example, the serialized value of
rgb
29
164
192
95
is the string "rgba(29, 164, 192, 0.95)"
For example, an author-supplied value:
hwb
740
deg
20
30
50
Would be normalized first to
hwb
20
20
30
50
and then converted to sRGB and serialized as
rgba
178.5
93.5
51
0.5
The precision of the returned result
is
described below
Note:
contrary to CSS Color 3,
the parameters of the
rgb()
function
are of type

, not

Thus, any higher precision than eight bits
is indicated with a fractional part.
The precision with which sRGB component values are retained,
and thus the number of significant figures in the serialized value,
is not defined in this specification,
but must at least be sufficient
to round-trip eight bit values.
Values must be
rounded towards +∞
, not truncated.
Note:
authors of scripts which expect
color values returned from
getComputedStyle
to have

component values,
are advised to update them to also cope with

For example,
rgb
146.064
107.457
131.223
is now valid, and equal to
rgb
57.28
42.14
51.46
A conformant serialized form for both,
is the string "rgb(146.06, 107.46, 131.2)".
Trailing fractional zeroes in any component values must be omitted;
if the fractional part consists of all zeroes,
the decimal point must also be omitted.
This means that sRGB colors specified with integer component values
will serialize with backwards-compatible integer values.
The serialized computed value of
''
goldenrod
''
is the string "rgb(218, 165, 32)"
and not the string "rgb(218.000, 165.000, 32.000)"
16.3.
Serializing Lab and LCH values
The serialized form of
lch()
and
lab()
values
is derived from the
computed value
and uses the
lab()
or
lch()
forms,
with
ASCII lowercase
letters for the function name.
The component values are serialized in base 10;
the L, a, b and C component values
are serialized as

using the
Lab percentage reference ranges
or the
LCH percentage reference ranges
as appropriate
to perform percentage to number conversion;
thus 0% L maps to 0
and 100% L maps to 100.
A single ASCII space character " "
must be used as the separator
between the component values.
Tests
color-computed.html
(live test)
(source)
The serialized value of
lab
56.200
0.000
83.600
is the string "lab(56.2 0 83.6)"
The serialized value of
lab
56.200
0.000
66.88
is the string "lab(56.2 0 83.6)"
Trailing fractional zeroes in any component values must be omitted;
if the fractional part consists of all zeroes,
the decimal point must also be omitted.
The serialized value of
lch
37
105.0
305.00
is the string "lch(37 105 305)",
not "lch(37 105.0 305.00)".
The precision with which
lab()
component values are retained,
and thus the number of significant figures in the serialized value,
is not defined in this specification,
but due to the wide gamut must be sufficient
to round-trip L values between 0 and 100,
and a and b values between ±127,
with at least sixteen bit precision;
this will result in at least three decimal places
unless trailing zeroes have been omitted.
(half float or float, is recommended for internal storage).
Values must be
rounded towards +∞
, not truncated.
Note:
a and b values outside ±125 are possible
with ultrawide gamut spaces. For example,
all
of the
prophoto-rgb
primaries and secondaries
exceed this range, but are within ±200.
As noted earlier
unitary alpha values are not explicitly serialized.
Non-unitary alpha values must be explicitly serialized,
and the string " / "
(an ASCII space, then forward slash, then another space)
must be used to separate the b component value from the alpha value.
The serialized value of
lch
56.2
83.6
357.4
93
is the string "lch(56.2 83.6 357.4 / 0.93)"
not "lch(56.2% 83.6 357.4 / 0.93)"
16.4.
Serializing Oklab and OkLCh values
The serialized form of
oklch()
and
oklab()
values
is derived from the
computed value
and uses the
oklab()
or
oklch()
forms,
with
ASCII lowercase
letters for the function name.
The component values are serialized in base 10;
the L, a, b and C component values
are serialized as

using the
Oklab percentage reference ranges
or the
OkLCh percentage reference ranges
as appropriate
to perform percentage to number conversion;
thus 0% L maps to 0
and 100% L maps to 1.0.
A single ASCII space character " "
must be used as the separator
between the component values.
Tests
color-computed.html
(live test)
(source)
The serialized value of
oklab
54.0
-0.10
-0.02
is the string "oklab(0.54 -0.1 -0.02)"
not "oklab(54 -0.1 -0.02)" or
"oklab(54% -0.1 -0.02)"
The serialized value of
oklab
54.0
-25
-5
is the string "oklab(0.54 -0.1 -0.02)"
not "oklab(54 -0.25 -0.05)"
Trailing fractional zeroes in any component values must be omitted;
if the fractional part consists of all zeroes,
the decimal point must also be omitted.
The serialized value of
oklch
56.43
0.0900
123.40
is the string "oklch(0.5643 0.09 123.4)",
not "oklch(0.5643 0.0900 123.40)".
The precision with which
oklab()
component values are retained,
and thus the number of significant figures in the serialized value,
is not defined in this specification,
but due to the wide gamut must be sufficient
to round-trip L values between 0 and 1 (0% and 100%),
and a, b and C values between ±0.5,
with at least sixteen bit precision;
this will result in at least five decimal places
unless trailing zeroes have been omitted.
(half float or float, is recommended for internal storage).
Values must be
rounded towards +∞
, not truncated.
Note:
a, b and C values outside ±0.5 are possible
with ultrawide gamut spaces. For example,
the
prophoto-rgb
green and blue primaries
exceed this range,
with C of 0.526 and 1.413 respectively.
As noted earlier
unitary alpha values are not explicitly serialized.
Non-unitary alpha values must be explicitly serialized,
and the string " / "
(an ASCII space, then forward slash, then another space)
must be used to separate the final color component (b, or C) value from the alpha value.
The serialized value of
oklch
53.85
0.1725
320.67
70
is the string "oklch(0.5385 0.1725 320.67 / 0.7)"
16.5.
Serializing values of the
color()
function
The serialized form of
color()
values
is derived from the
computed value
and uses the
color()
form,
with
ASCII lowercase
letters for the function name
and the color space name.
The component values are serialized in base 10,
as

A single ASCII space character " "
must be used as the separator
between the component values,
and also between the color space name and the first color component.
Tests
color-computed.html
(live test)
(source)
The serialized value of
color
dIsPlAy-P3
0.964
0.763
0.787
is the string "color(display-p3 0.96 0.76 0.79)",
if two decimal places are retained.
Notice that 0.787 has rounded up to 0.79,
rather than being truncated to 0.78.
Trailing fractional zeroes in any component values must be omitted;
if the fractional part consists of all zeroes,
the decimal point must also be omitted.
The serialized value of
color
rec2020
0.400
0.660
0.340
is the string "color(rec2020 0.4 0.66 0.34)",
not "color(rec2020 0.400 0.660 0.340)".
If the color space is sRGB, the color space is still explicitly required in the serialized result.
For the predefined color spaces,
the
minimum
precision for round-tripping is as follows:
color space
Minimum bits
srgb
10
srgb-linear
12
display-p3
10
display-p3-linear
12
a98-rgb
10
prophoto-rgb
12
rec2020
12
xyz
xyz-d50
xyz-d65
16
(16bit, half-float, or float
per component
is recommended for internal storage).
Values must be
rounded towards +∞
, not truncated.
Note:
compared to the legacy forms
such as
rgb()
hsl()
and so on,
color(srgb)
has a higher minimum precision requirement.
Stylesheet authors who prefer higher precision
are thus encouraged to use the
color(srgb)
form.
As noted earlier
unitary alpha values are not explicitly serialized.
Non-unitary alpha values must be explicitly serialized,
and the string " / "
(an ASCII space, then forward slash, then another space)
must be used to separate
the final color component value
from the alpha value.
The serialized value of
color
prophoto-rgb
0.2804
0.40283
0.42259
85
is the string "color(prophoto-rgb 0.28 0.403 0.423 / 0.85)",
if three decimal places are retained.
16.6.
Serializing other colors
This applies to
currentcolor
The serialized form of this value
is derived from the
computed value
and uses
ASCII lowercase
letters for the color name.
The serialized form of
currentColor
is the string "currentcolor".
17.
Serializing

This applies to the
opacity
property.
If the specified value for an opacity value
matches a literal

(i.e. does not use
calc()
it should be serialized as the equivalent

(0% maps to 0, 100% maps to 1) value.
value.
Otherwise, the specified value
for an opacity value
should serialize using
the standard serialization for the grammar.
This

value is expressed in base ten,
with the "." character as decimal separator.
The leading zero must not be omitted.
Trailing zeroes must be omitted.
Opacity values outside the range [0,1]
are preserved, without clamping, in the serialized specified value.
The precision with which opacity values are retained,
and thus the number of decimal places in the serialized value,
is not defined in this specification,
but must at least be sufficient
to round-trip integer percentage values.
Thus, the serialized value must contain
at least two decimal places
(unless trailing zeroes have been removed).
Values must be
rounded
towards +∞
, not truncated.
18.
Default Style Rules
The following stylesheet is informative, not normative. This style sheet could be used by an implementation as part of its default styling of HTML documents.
/* traditional desktop user agent colors for hyperlinks */
:link
color
LinkText
:visited
color
VisitedText
:active
color
ActiveText
19.
Sample code for Color Conversions
This section is not normative.
Tests
This section is not normative, it does not need tests.
For clarity,
a library
is used for matrix multiplication.
(This is more readable than inlining all the multiplies and adds).
The matrices are in
column-major order
// Sample code for color conversions
// Conversion can also be done using ICC profiles and a Color Management System
// For clarity, a library is used for matrix multiplication (multiply-matrices.js)
// standard white points, defined by 4-figure CIE x,y chromaticities
const
D50
0.3457
0.3585
1.00000
1.0
0.3457
0.3585
0.3585
];
const
D65
0.3127
0.3290
1.00000
1.0
0.3127
0.3290
0.3290
];
// sRGB-related functions
function
lin_sRGB
RGB
// convert an array of sRGB values
// where in-gamut values are in the range [0 - 1]
// to linear light (un-companded) form.
// https://en.wikipedia.org/wiki/SRGB
// Extended transfer function:
// for negative values, linear portion is extended on reflection of axis,
// then reflected power function is used.
return
RGB
map
function
val
let
sign
val
let
abs
Math
abs
val
);
if
abs
<=
0.04045
return
val
12.92
return
sign
Math
pow
((
abs
0.055
1.055
2.4
));
});
function
gam_sRGB
RGB
// convert an array of linear-light sRGB values in the range 0.0-1.0
// to gamma corrected form
// https://en.wikipedia.org/wiki/SRGB
// Extended transfer function:
// For negative values, linear portion extends on reflection
// of axis, then uses reflected pow below that
return
RGB
map
function
val
let
sign
val
let
abs
Math
abs
val
);
if
abs
0.0031308
return
sign
1.055
Math
pow
abs
2.4
0.055
);
return
12.92
val
});
function
lin_sRGB_to_XYZ
rgb
// convert an array of linear-light sRGB values to CIE XYZ
// using sRGB's own white, D65 (no chromatic adaptation)
var
506752
1228815
87881
245763
12673
70218
],
87098
409605
175762
245763
12673
175545
],
7918
409605
87881
737289
1001167
1053270
],
];
return
multiplyMatrices
rgb
);
function
XYZ_to_lin_sRGB
XYZ
// convert XYZ to linear-light sRGB
var
12831
3959
329
214
1974
3959
],
851781
878810
1648619
878810
36519
878810
],
705
12673
2585
12673
705
667
],
];
return
multiplyMatrices
XYZ
);
// display-p3-related functions
function
lin_P3
RGB
// convert an array of display-p3 RGB values in the range 0.0 - 1.0
// to linear light (un-companded) form.
return
lin_sRGB
RGB
);
// same as sRGB
function
gam_P3
RGB
// convert an array of linear-light display-p3 RGB in the range 0.0-1.0
// to gamma corrected form
return
gam_sRGB
RGB
);
// same as sRGB
function
lin_P3_to_XYZ
rgb
// convert an array of linear-light display-p3 values to CIE XYZ
// using D65 (no chromatic adaptation)
// http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html
var
608311
1250200
189793
714400
198249
1000160
],
35783
156275
247089
357200
198249
2500400
],
32229
714400
5220557
5000800
],
];
return
multiplyMatrices
rgb
);
function
XYZ_to_lin_P3
XYZ
// convert XYZ to linear-light P3
var
446124
178915
333277
357830
72051
178915
],
14852
17905
63121
35810
423
17905
],
11844
330415
50337
660830
316169
330415
],
];
return
multiplyMatrices
XYZ
);
// prophoto-rgb functions
function
lin_ProPhoto
RGB
// convert an array of prophoto-rgb values
// where in-gamut colors are in the range [0.0 - 1.0]
// to linear light (un-companded) form.
// Transfer curve is gamma 1.8 with a small linear portion
// Extended transfer function
const
Et2
16
512
return
RGB
map
function
val
let
sign
val
let
abs
Math
abs
val
);
if
abs
<=
Et2
return
val
16
return
sign
Math
pow
abs
1.8
);
});
function
gam_ProPhoto
RGB
// convert an array of linear-light prophoto-rgb in the range 0.0-1.0
// to gamma corrected form
// Transfer curve is gamma 1.8 with a small linear portion
// TODO for negative values, extend linear portion on reflection of axis, then add pow below that
const
Et
512
return
RGB
map
function
val
let
sign
val
let
abs
Math
abs
val
);
if
abs
>=
Et
return
sign
Math
pow
abs
1.8
);
return
16
val
});
function
lin_ProPhoto_to_XYZ
rgb
// convert an array of linear-light prophoto-rgb values to CIE D50 XYZ
// matrix cannot be expressed in rational form, but is calculated to 64 bit accuracy
// see https://github.com/w3c/csswg-drafts/issues/7675
var
0.79776664490064230
0.13518129740053308
0.03134773412839220
],
0.28807482881940130
0.71183523424187300
0.00008993693872564
],
0.00000000000000000
0.00000000000000000
0.82510460251046020
];
return
multiplyMatrices
rgb
);
function
XYZ_to_lin_ProPhoto
XYZ
// convert D50 XYZ to linear-light prophoto-rgb
var
1.34578688164715830
0.25557208737979464
0.05110186497554526
],
0.54463070512490190
1.50824774284514680
0.02052744743642139
],
0.00000000000000000
0.00000000000000000
1.21196754563894520
];
return
multiplyMatrices
XYZ
);
// a98-rgb functions
function
lin_a98rgb
RGB
// convert an array of a98-rgb values in the range 0.0 - 1.0
// to linear light (un-companded) form.
// negative values are also now accepted
return
RGB
map
function
val
let
sign
val
let
abs
Math
abs
val
);
return
sign
Math
pow
abs
563
256
);
});
function
gam_a98rgb
RGB
// convert an array of linear-light a98-rgb in the range 0.0-1.0
// to gamma corrected form
// negative values are also now accepted
return
RGB
map
function
val
let
sign
val
let
abs
Math
abs
val
);
return
sign
Math
pow
abs
256
563
);
});
function
lin_a98rgb_to_XYZ
rgb
// convert an array of linear-light a98-rgb values to CIE XYZ
// http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html
// has greater numerical precision than section 4.3.5.3 of
// https://www.adobe.com/digitalimag/pdfs/AdobeRGB1998.pdf
// but the values below were calculated from first principles
// from the chromaticity coordinates of R G B W
// see matrixmaker.html
var
573536
994567
263643
1420810
187206
994567
],
591459
1989134
6239551
9945670
374412
4972835
],
53769
1989134
351524
4972835
4929758
4972835
],
];
return
multiplyMatrices
rgb
);
function
XYZ_to_lin_a98rgb
XYZ
// convert XYZ to linear-light a98-rgb
var
1829569
896150
506331
896150
308931
896150
],
851781
878810
1648619
878810
36519
878810
],
16779
1248040
147721
1248040
1266979
1248040
],
];
return
multiplyMatrices
XYZ
);
//Rec. 2020-related functions
function
lin_2020
RGB
// convert an array of rec2020 RGB values in the range 0.0 - 1.0
// to linear light (un-companded) form.
// Reference electro-optical transfer function from Rec. ITU-R BT.1886 Annex 1
// with b (black lift) = 0 and a (user gain) = 1
// defined over the extended range, not clamped
return
RGB
map
function
val
let
sign
val
let
abs
Math
abs
val
);
return
sign
Math
pow
abs
2.4
);
});
function
gam_2020
RGB
// convert an array of linear-light rec2020 RGB in the range 0.0-1.0
// to gamma corrected form
// Reference electro-optical transfer function from Rec. ITU-R BT.1886 Annex 1
// with b (black lift) = 0 and a (user gain) = 1
// defined over the extended range, not clamped
return
RGB
map
function
val
let
sign
val
let
abs
Math
abs
val
);
return
sign
Math
pow
abs
2.4
);
});
function
lin_2020_to_XYZ
rgb
// convert an array of linear-light rec2020 values to CIE XYZ
// using D65 (no chromatic adaptation)
var
63426534
99577255
20160776
139408157
47086771
278816314
],
26158966
99577255
472592308
697040785
8267143
139408157
],
19567812
697040785
295819943
278816314
],
];
// 0 is actually calculated as 4.994106574466076e-17
return
multiplyMatrices
rgb
);
function
XYZ_to_lin_2020
XYZ
// convert XYZ to linear-light rec2020
var
30757411
17917100
6372589
17917100
4539589
17917100
],
19765991
29648200
47925759
29648200
467509
29648200
],
792561
44930125
1921689
44930125
42328811
44930125
],
];
return
multiplyMatrices
XYZ
);
// Chromatic adaptation
function
D65_to_D50
XYZ
// Bradford chromatic adaptation from D65 to D50
// The matrix below is the result of three operations:
// - convert from XYZ to retinal cone domain
// - scale components from one reference white to another
// - convert back to XYZ
// see https://github.com/LeaVerou/color.js/pull/354/files
var
1.0479297925449969
0.022946870601609652
0.05019226628920524
],
0.02962780877005599
0.9904344267538799
0.017073799063418826
],
0.009243040646204504
0.015055191490298152
0.7518742814281371
];
return
multiplyMatrices
XYZ
);
function
D50_to_D65
XYZ
// Bradford chromatic adaptation from D50 to D65
// See https://github.com/LeaVerou/color.js/pull/360/files
var
0.955473421488075
0.02309845494876471
0.06325924320057072
],
0.0283697093338637
1.0099953980813041
0.021041441191917323
],
0.012314014864481998
0.020507649298898964
1.330365926242124
];
return
multiplyMatrices
XYZ
);
// CIE Lab and LCH
function
XYZ_to_Lab
XYZ
// Assuming XYZ is relative to D50, convert to CIE Lab
// from CIE standard, which now defines these as a rational fraction
var
216
24389
// 6^3/29^3
var
24389
27
// 29^3/3^3
// compute xyz, which is XYZ scaled relative to reference white
var
xyz
XYZ
map
((
value
=>
value
D50
]);
// now compute f
var
xyz
map
value
=>
value
Math
cbrt
value
value
16
116
);
return
116
])
16
// L
500
]),
// a
200
])
// b
];
// L in range [0,100]. For use in CSS, add a percent
function
Lab_to_XYZ
Lab
// Convert Lab to D50-adapted XYZ
// http://www.brucelindbloom.com/index.html?Eqn_Lab_to_XYZ.html
var
24389
27
// 29^3/3^3
var
216
24389
// 6^3/29^3
var
[];
// compute f, starting with the luminance-related term
Lab
16
116
Lab
500
];
Lab
200
// compute xyz
var
xyz
Math
pow
],
Math
pow
],
116
16
Lab
Math
pow
((
Lab
16
116
Lab
Math
pow
],
Math
pow
],
116
16
];
// Compute XYZ by scaling xyz by reference white
return
xyz
map
((
value
=>
value
D50
]);
function
Lab_to_LCH
Lab
var
epsilon
0.0015
var
chroma
Math
sqrt
Math
pow
Lab
],
Math
pow
Lab
],
));
// Chroma
var
hue
Math
atan2
Lab
],
Lab
])
180
Math
PI
if
hue
hue
hue
360
if
chroma
<=
epsilon
hue
NaN
return
Lab
],
// L is still L
chroma
// Chroma
hue
// Hue, in degrees [0 to 360)
];
function
LCH_to_Lab
LCH
// Convert from polar form
return
LCH
],
// L is still L
LCH
Math
cos
LCH
Math
PI
180
),
// a
LCH
Math
sin
LCH
Math
PI
180
// b
];
// OKLab and OKLCH
// https://bottosson.github.io/posts/oklab/
// XYZ <-> LMS matrices recalculated for consistent reference white
// see https://github.com/w3c/csswg-drafts/issues/6642#issuecomment-943521484
// recalculated for 64bit precision
// see https://github.com/color-js/color.js/pull/357
function
XYZ_to_OKLab
XYZ
// Given XYZ relative to D65, convert to OKLab
var
XYZtoLMS
0.8190224379967030
0.3619062600528904
0.1288737815209879
],
0.0329836539323885
0.9292868615863434
0.0361446663506424
],
0.0481771893596242
0.2642395317527308
0.6335478284694309
];
var
LMStoOKLab
0.2104542683093140
0.7936177747023054
0.0040720430116193
],
1.9779985324311684
2.4285922420485799
0.4505937096174110
],
0.0259040424655478
0.7827717124575296
0.8086757549230774
];
var
LMS
multiplyMatrices
XYZtoLMS
XYZ
);
// JavaScript Math.cbrt returns a sign-matched cube root
// beware if porting to other languages
// especially if tempted to use a general power function
return
multiplyMatrices
LMStoOKLab
LMS
map
=>
Math
cbrt
)));
// L in range [0,1]. For use in CSS, multiply by 100 and add a percent
function
OKLab_to_XYZ
OKLab
// Given OKLab, convert to XYZ relative to D65
var
LMStoXYZ
1.2268798758459243
0.5578149944602171
0.2813910456659647
],
0.0405757452148008
1.1122868032803170
0.0717110580655164
],
0.0763729366746601
0.4214933324022432
1.5869240198367816
];
var
OKLabtoLMS
1.0000000000000000
0.3963377773761749
0.2158037573099136
],
1.0000000000000000
0.1055613458156586
0.0638541728258133
],
1.0000000000000000
0.0894841775298119
1.2914855480194092
];
var
LMSnl
multiplyMatrices
OKLabtoLMS
OKLab
);
return
multiplyMatrices
LMStoXYZ
LMSnl
map
=>
**
));
function
OKLab_to_OKLCH
OKLab
var
epsilon
0.000004
var
hue
Math
atan2
OKLab
],
OKLab
])
180
Math
PI
var
chroma
Math
sqrt
OKLab
**
OKLab
**
);
if
hue
hue
hue
360
if
chroma
<=
epsilon
hue
NaN
return
OKLab
],
// L is still L
chroma
hue
];
function
OKLCH_to_OKLab
OKLCH
return
OKLCH
],
// L is still L
OKLCH
Math
cos
OKLCH
Math
PI
180
),
// a
OKLCH
Math
sin
OKLCH
Math
PI
180
// b
];
// Premultiplied alpha conversions
function
rectangular_premultiply
color
alpha
// given a color in a rectangular orthogonal colorspace
// and an alpha value
// return the premultiplied form
return
color
map
((
=>
alpha
function
rectangular_un_premultiply
color
alpha
// given a premultiplied color in a rectangular orthogonal colorspace
// and an alpha value
// return the actual color
if
alpha
===
return
color
// avoid divide by zero
return
color
map
((
=>
alpha
function
polar_premultiply
color
alpha
hueIndex
// given a color in a cylindicalpolar colorspace
// and an alpha value
// return the premultiplied form.
// the index says which entry in the color array corresponds to hue angle
// for example, in OKLCH it would be 2
// while in HSL it would be 0
return
color
map
((
=>
hueIndex
===
alpha
))
function
polar_un_premultiply
color
alpha
hueIndex
// given a color in a cylindicalpolar colorspace
// and an alpha value
// return the actual color.
// the hueIndex says which entry in the color array corresponds to hue angle
// for example, in OKLCH it would be 2
// while in HSL it would be 0
if
alpha
===
return
color
// avoid divide by zero
return
color
map
((
=>
hueIndex
===
alpha
))
// Convenience functions can easily be defined, such as
function
hsl_premultiply
color
alpha
return
polar_premultiply
color
alpha
);
20.
Sample Code for ΔE2000 and ΔEOK Color Differences
This section is not normative.
Tests
This section is not normative, it does not need tests.
20.1.
ΔE2000
The simplest color difference metric, ΔE76,
is simply the Euclidean distance in Lab color space.
While this is a good first approximation,
color-critical industries such as printing and fabric dyeing
soon developed improved formulae.
Currently, the most widely used formula
is ΔE2000.
It corrects a number of known asymmetries and non-linearities
compared to ΔE76.
Because the formula is complex,
and critically dependent on the sign
of various intermediate calculations,
implementations are often incorrect
[Sharma]
The sample code below has been
validated
to five significant figures
against the test suite of paired Lab values and expected ΔE2000
published by
[Sharma]
and is correct.
// deltaE2000 is a statistically significant improvement
// over deltaE76 and deltaE94,
// and is recommended by the CIE and Idealliance
// especially for color differences less than 10 deltaE76
// but is wicked complicated
// and many implementations have small errors!
/**
* @param {number[]} reference - Array of CIE Lab values: L as 0..100, a and b as around -150..150
* @param {number[]} sample - Array of CIE Lab values: L as 0..100, a and b as around -150..150
* @return {number} How different a color sample is from reference
*/
function
deltaE2000
reference
sample
// Given a reference and a sample color,
// both in CIE Lab,
// calculate deltaE 2000.
// This implementation assumes the parametric
// weighting factors kL, kC and kH
// (for the influence of viewing conditions)
// are all 1, as seems typical.
let
L1
a1
b1
reference
let
L2
a2
b2
sample
let
C1
Math
sqrt
a1
**
b1
**
);
let
C2
Math
sqrt
a2
**
b2
**
);
let
Cbar
C1
C2
// mean Chroma
// calculate a-axis asymmetry factor from mean Chroma
// this turns JND ellipses for near-neutral colors back into circles
let
C7
Math
pow
Cbar
);
const
Gfactor
Math
pow
25
);
let
0.5
Math
sqrt
C7
C7
Gfactor
)));
// scale a axes by asymmetry factor
// this by the way is why there is no Lab2000 color space
let
adash1
a1
let
adash2
a2
// calculate new Chroma from scaled a and original b axes
let
Cdash1
Math
sqrt
adash1
**
b1
**
);
let
Cdash2
Math
sqrt
adash2
**
b2
**
);
// calculate new hues, with zero hue for true neutrals
// and in degrees, not radians
const
Math
PI
const
r2d
180
const
d2r
180
let
h1
adash1
===
&&
b1
===
Math
atan2
b1
adash1
);
let
h2
adash2
===
&&
b2
===
Math
atan2
b2
adash2
);
if
h1
h1
+=
if
h2
h2
+=
h1
*=
r2d
h2
*=
r2d
// Lightness and Chroma differences; sign matters
let
ΔL
L2
L1
let
ΔC
Cdash2
Cdash1
// Hue difference, taking care to get the sign correct
let
hdiff
h2
h1
let
hsum
h1
h2
let
habs
Math
abs
hdiff
);
let
Δh
if
Cdash1
Cdash2
===
Δh
else
if
habs
<=
180
Δh
hdiff
else
if
hdiff
180
Δh
hdiff
360
else
if
hdiff
180
Δh
hdiff
360
else
console
log
"the unthinkable has happened"
);
// weighted Hue difference, more for larger Chroma
let
ΔH
Math
sqrt
Cdash2
Cdash1
Math
sin
Δh
d2r
);
// calculate mean Lightness and Chroma
let
Ldash
L1
L2
let
Cdash
Cdash1
Cdash2
let
Cdash7
Math
pow
Cdash
);
// Compensate for non-linearity in the blue region of Lab.
// Four possibilities for hue weighting factor,
// depending on the angles, to get the correct sign
let
hdash
if
Cdash1
Cdash2
===
hdash
hsum
else
if
habs
<=
180
hdash
hsum
else
if
hsum
360
hdash
hsum
360
else
hdash
hsum
360
// positional corrections to the lack of uniformity of CIELAB
// These are all trying to make JND ellipsoids more like spheres
// SL Lightness crispening factor
// a background with L=50 is assumed
let
lsq
Ldash
50
**
let
SL
((
0.015
lsq
Math
sqrt
20
lsq
));
// SC Chroma factor, similar to those in CMC and deltaE 94 formulae
let
SC
0.045
Cdash
// Cross term T for blue non-linearity
let
-=
0.17
Math
cos
((
hdash
30
d2r
));
+=
0.24
Math
cos
hdash
d2r
));
+=
0.32
Math
cos
(((
hdash
d2r
));
-=
0.20
Math
cos
(((
hdash
63
d2r
));
// SH Hue factor depends on Chroma,
// as well as adjusted hue angle like deltaE94.
let
SH
0.015
Cdash
// RT Hue rotation term compensates for rotation of JND ellipses
// and Munsell constant hue lines
// in the medium-high Chroma blue region
// (Hue 225 to 315)
let
Δθ
30
Math
exp
(((
hdash
275
25
**
));
let
RC
Math
sqrt
Cdash7
Cdash7
Gfactor
));
let
RT
Math
sin
Δθ
d2r
RC
// Finally calculate the deltaE, term by term as root sum of squares
let
dE
ΔL
SL
**
dE
+=
ΔC
SC
**
dE
+=
ΔH
SH
**
dE
+=
RT
ΔC
SC
ΔH
SH
);
return
Math
sqrt
dE
);
// Yay!!!
};
20.2.
ΔEOK
Because Oklab does not suffer from
the hue linearity, hue uniformity,
and chroma non-linearities of CIE Lab,
the color difference metric does not need to correct for them
and so
is simply the Euclidean distance in Oklab color space.
// Calculate deltaE OK
// simple root sum of squares
/**
* @param {number[]} reference - Array of OKLab values: L as 0..1, a and b as -1..1
* @param {number[]} sample - Array of OKLab values: L as 0..1, a and b as -1..1
* @return {number} How different a color sample is from reference
*/
function
deltaEOK
reference
sample
let
L1
a1
b1
reference
let
L2
a2
b2
sample
let
ΔL
L1
L2
let
Δa
a1
a2
let
Δb
b1
b2
return
Math
sqrt
ΔL
**
Δa
**
Δb
**
);
Appendix A: Deprecated CSS System Colors
Earlier versions of CSS defined several additional
system colors
These color keywords have been
deprecated
, however,
as they are insufficient for their original purpose
(making website elements look like their native OS counterparts),
represent a security risk
by making it easier for a webpage to “spoof” a native OS dialog,
and increase fingerprinting surface, compromising user privacy.
User agents must support these keywords,
and to mitigate fingerprinting
must map them to the (undeprecated)
system colors
as listed below.
Authors must not use these keywords.
The deprecated system colors are represented
as the

sub-type,
and are defined as:
ActiveBorder
Active window border. Same as
ButtonBorder
ActiveCaption
Active window caption. Same as
Canvas
AppWorkspace
Background color of multiple document interface. Same as
Canvas
Background
Desktop background. Same as
Canvas
ButtonHighlight
The color of the border facing the light source for 3-D elements
that appear 3-D due to one layer of surrounding border. Same as
ButtonFace
ButtonShadow
The color of the border away from the light source for 3-D elements
that appear 3-D due to one layer of surrounding border. Same as
ButtonFace
CaptionText
Text in caption, size box, and scrollbar arrow box. Same as
CanvasText
InactiveBorder
Inactive window border. Same as
ButtonBorder
InactiveCaption
Inactive window caption. Same as
Canvas
InactiveCaptionText
Color of text in an inactive caption. Same as
GrayText
InfoBackground
Background color for tooltip controls. Same as
Canvas
InfoText
Text color for tooltip controls. Same as
CanvasText
Menu background. Same as
Canvas
MenuText
Text in menus. Same as
CanvasText
Scrollbar
Scroll bar gray area. Same as
Canvas
ThreeDDarkShadow
The color of the darker (generally outer) of the two borders away
from the light source for 3-D elements that appear 3-D due to two
concentric layers of surrounding border. Same as
ButtonBorder
ThreeDFace
The face background color for 3-D elements that appear 3-D due to
two concentric layers of surrounding border. Same as
ButtonFace
ThreeDHighlight
The color of the lighter (generally outer) of the two borders facing
the light source for 3-D elements that appear 3-D due to two
concentric layers of surrounding border. Same as
ButtonBorder
ThreeDLightShadow
The color of the darker (generally inner) of the two borders facing
the light source for 3-D elements that appear 3-D due to two
concentric layers of surrounding border. Same as
ButtonBorder
ThreeDShadow
The color of the lighter (generally inner) of the two borders away
from the light source for 3-D elements that appear 3-D due to two
concentric layers of surrounding border. Same as
ButtonBorder
Window
Window background. Same as
Canvas
WindowFrame
Window frame. Same as
ButtonBorder
WindowText
Text in windows. Same as
CanvasText
Tests
deprecated-sameas-001.html
(live test)
(source)
deprecated-sameas-002.html
(live test)
(source)
deprecated-sameas-003.html
(live test)
(source)
deprecated-sameas-004.html
(live test)
(source)
deprecated-sameas-005.html
(live test)
(source)
deprecated-sameas-006.html
(live test)
(source)
deprecated-sameas-007.html
(live test)
(source)
deprecated-sameas-008.html
(live test)
(source)
deprecated-sameas-009.html
(live test)
(source)
deprecated-sameas-010.html
(live test)
(source)
deprecated-sameas-011.html
(live test)
(source)
deprecated-sameas-012.html
(live test)
(source)
deprecated-sameas-013.html
(live test)
(source)
deprecated-sameas-014.html
(live test)
(source)
deprecated-sameas-015.html
(live test)
(source)
deprecated-sameas-016.html
(live test)
(source)
deprecated-sameas-017.html
(live test)
(source)
deprecated-sameas-018.html
(live test)
(source)
deprecated-sameas-019.html
(live test)
(source)
deprecated-sameas-020.html
(live test)
(source)
deprecated-sameas-021.html
(live test)
(source)
deprecated-sameas-022.html
(live test)
(source)
deprecated-sameas-023.html
(live test)
(source)
Appendix B: Deprecated Quirky Hex Colors
When CSS is being parsed in
quirks mode

is a type of

that is only valid in certain properties:
background-color
border-color
border-top-color
border-right-color
border-bottom-color
border-left-color
color
It is
not
valid in properties that include or reference these properties,
such as the
background
shorthand,
or inside
functional notations
such as
color-mix()
Additionally, while

must be valid as a

when parsing the affected properties in the
@supports
rule,
it is
not
valid for those properties
when used in the
CSS.supports()
method.

can be represented
as a


, or

according to the following rules:
If it’s an

the token’s representation must contain exactly 3 or 6 characters,
all hexadecimal digits.
It represents a

with the same value.
If it’s a

it must have its integer flag set.
Serialize the integer’s value.
If the serialization has less than 6 characters,
prepend "0" characters to it until it is 6 characters long.
It represents a

with the same value.
If it’s a

it must have its integer flag set.
Serialize the integer’s value,
and append the representation of the token’s unit.
If the result has less than 6 characters,
prepend "0" characters to it until it is 6 characters long.
It represents a

with the same value.
(In other words, Quirks Mode allows hex colors to be written without the leading "#",
but with weird parsing rules.)
Tests
quirky hex colors
Acknowledgments
In addition to
those
who contributed to CSS Color 3
, the editors would like to thank
Emilio Cobos Álvarez, Alexey Ardov, Chris Bai, Amelia Bellamy-Royds, Lars Borg, Mike Bremford, Andreu Botella, Dan Burzo, Max Derhak, fantasai, Simon Fraser, Devon Govett, Phil Green, Dean Jackson, Andreas Kraushaar, Pierre-Anthony Lemieux, Tiaan Louw, Cameron McCormack, Romain Menke, Chris Murphy, Isaac Muse, Jonathan Neal, Chris Needham, Björn Ottosson, Christoph Päper, Brad Pettit, Xidorn Quan, Craig Revie, Melanie Richards, Florian Rivoal, Jacob Rus, Joseph Salowey, Simon Sapin, Igor Snitkin, Lea Verou, Mark Watson, James Stuckey Weber, Sam Weinig, and Natalie Weizenbaum.
Changes
Changes since the
Candidate Recommendation Draft of 24 April 2025
Added a new section defining when two

values are
equivalent colors
covering same-color-space component comparison, the treatment of
missing component
s,
and cross-color-space comparison via
oklab
Issue 13157
Clarified in the main Color interpolation section that, if the hue interpolaton method is not specified, shorter is the default. (This was already specified in the Hue Interpolation section).
Issue 13788
Expanded the concept of analogous components to analogous sets of components, to minimize
none
conversions
Issue 10210
Split color conversion into two stages
Issue 10211
Clarified how system colors react to the used color scheme
Issue 13719
Updated abstract to mention color interpolation and gamut mapping.
Clarified wording regarding the aims of CSS gamut mapping
Corrected ray trace algorithm to not overwrite
end
Issue 10579
Added pseudocode for the ray trace gamut mapping algorithm
Issue 10579
Added EdgeSeeker and Ray Trace Gamut Mapping Algorithms. Allowed choice of three GMA
Issue 10579
Added a diagram showing imaginary colors in CIE Lab
Differentiated between out of gamut but physically realizable colors, and imaginary colors
More even-handed description of clipping,
showing some cases which give acceptable results
Issue 10579
Fixed discrepency in ΔE2000 sample implementation
Issue 13322
Updated
AccentColor
to take its value from
accent-color
, unless in
Forced Colors Mode
Issue 5900
Defined rec2020 color space to use display-referred, 2.4 gamma
Issue 12574
Added display-p3-linear to predefined colorspaces
Issue 11250
Clarified serializing opacity values with calc()
Issue 10426
Interpolation between legacy sRGB colors is (once again) in sRGB space, for compatibility
Issue 7949
Clarified real-world CIE Lab range for a and b
Issue 12208
Clarified that Opacity value does not affect hit testing
Issue 11339
Changes since the
Candidate Recommendation Draft of 13 Feb 2024
Clarified that inside the color property, it is the resolved inherited value (not the raw inherited value) that is used
Listed categories of colors, such as those that resolve to sRGB or support legacy color syntax
Added hue normalization examples
Corrected table of analogous components, alpha was missing, but described as analogous in prose
Collected together and clarified serialization of opacity values into one section
Clarified wording around color-interpolation-method and host syntax
Defined epsilon for returning missing hue
Used a more precise definition of achromatic colors with missing hues (sufficiently close to the central axis)
Consistently use "color component" rather than "color channel" (both were being used).
Correlated Color Temperature was used without being defined or explained. Added informative reference.
Exported term premultiplied, linked to it consistently
Equivalence of deprecated and un-deprecated system colors is no longer at-risk
Clarified intended use of the "parse a CSS

" algorithm
Added corrected examples for HTML-compatible serialization
Removed check on missing values for HTML-compatible serialization, they will already have been converted to zero
Moved note about missing values becoming 0, so it applies to both HTML-compatible and CSS serializations
Added an HTML-Compatible hex serialization for sRGB
Added another xyz-d65 and xyz-d50 example
Clarify which component (Y) in XYZ corresponds to brightness
Clarified that CSS gamut mapping applies on actual, not used, values
Removed hue normalization from the hslToRgb sample code, as the input is already normalized at parse time
Corrected the pseudo-code for step 4 of the gamut mapping algorithm
Clarified that interpolation is the most common situation which combines two colors, but not the only one.
Ensured adequate contrast for text in the deltaE table
Removed remaining use of the term , use function instead for consistency
Updated acknowledgements section
Added gamut mesh diagram
Described CSSOM serialization in terms of declared values rather than specified values
Added exported definition of luminance
Added production rule for
Clarified when results from hslToRgb will be in [0,1]
Clarified that once linearized, RGB spaces are additive
Changes since the
Candidate Recommendation Draft of 1 November 2022
Added steps for serializing a uint8_t alpha, moved from cssom-1
Restored parse-time clamping of HSL negative saturation to 0, which is current interop behavior from CSS Color 3
When interpolating, always convert color space, so that powerless components become missing
Clarified when alpha 1 is omitted from serialization
Removed redundant constraining of hue angles to [0,360] as this is already done.
Corrected description of ActiveCaption, which is a background.
Disambiguated opacity and alpha. Opacity property now uses opacity-value (which has different clamping behavior to alpha-value)
Clarified that carrying forward happens before premultiplication
Updated gamut mapping algorithm
Fixed a few issues regarding hue interpolation
Clarified that HWB white or black at 100% is insufficient criterion for an achromatic color; it is the sum which matters.
Avoid returning negative saturation in rgb to hsl conversion; adjust the hue to point to "the other side" instead
Use 64 bit accurate matrices for ProPhoto, which does not have a rational form
Oklab matrices recalculated for 64bit precision (returns same results as before, at 32 bit precision)
Consistently return output of GMA in the destination color space, even if no mapping is performed because the destination is unbounded
Added explanation for why one JND for Oklab is 0.02, not 2
Clarified that resolving sRGB values does not apply to the color() function
Moved alpha value definition up to the opacity property, clarified that opacity specified values are not clamped.
System Colors now explicitly permit spoofing, to preserve privacy
Corrected the inverse chromatic adaptation matrix for D50 to D65
Consistently distinguish linear Bradfrod from the original, more complicated, Bradford chromatic adaptation algorithm
In the gamut mapping algorithm, return clipped as the gamut mapped result, avoiding un-necessary steps
Updated chromatic adaptation matrices to higher precision
Added Add 'parse a css color' algorithm, so non-CSS specs using colors don’t have to reinvent the machinery here.
Clarified that geometric gamut mapping must not project chroma back beyond the original color
Use the term "geometric" rather than "analytical" in gamut mapping discussion
Aligned prose for HSL into line with the grammar (percent and number both allowed)
Fixed an LCH alpha interpolation example, which was erroneously un-premultiplying the hue angle
Corrected the sRGB and display-p3 transfer function. (This only affected the result if a component had the exact value 10.31475 / 255, which is not possible at 8 or 10 bits per component)
Clarified that the specified values of system colors are still themselves
Added mention of PNG cICP chunk for tagging images
Described behaviour of hue increasing and decreasing when 0/360 is passed
Aligned description of powerlessnes in HSL with the other polar color models
Explicitly defined order of operations for color interpolation
Added mention of degenerate numeric constants in calc()
Clarified that calc() in sRGB has early resolution, and clamps the result
Clarified that HWB hue has the same disadvantages as HSL hue
Added luminance to lightness comparison and figure
Added descriptions and examples for hue interpolation keywords
Use normative prose for achromatic HWB colors
Corrected hue interpolation angle range; [0,360) not [0,360]
Expressed that displaying as black or white when L=0% or 100% is due to gamut mapping. Removed incorrect assertions of powerlessness
Dropped the confusing "representing black" and "representing white" comments
Clarified that opponent a and b are analogous
Specified RGB components using reference ranges rather than prose, for consistency
Explicitly referenced percent reference ranges for percentage to number conversion when serializing Lab, LCH, Oklab, OkLCh
Required Oklab interpolation, remove previous "may", describe explicit opt-out
Labelled the Lab, LCH, Oklab and OkLCh tutorial sections as non-normative. Moved some definitions out of the non-normative section.
Clarified that, when interpolating, checking for analogous components happens before color space conversion
Back-ported hwb() syntax changes and reference ranges from CSS Color 5
Defined carry-forward operations must happen before powerless operations
Clarified it is
color
components which must be all-number or all-percentage, in legacy rgb() syntax
Clarified for legacy syntax that color components must be all-percentage or all-number
Added examples of specified out of range alpha, with and without calc()
Placed examples of serializing with trimmed trailing zeroes colorer to the relevant text
clarified example, used value of text-shadow
Clarified resolving currentColor
Updated acknowledgments
Stop claiming that achromatic colors have missing a,b, or chroma
HSL and HWB changed to unbounded gamut, to promote round-tripping
Defined percentage reference range for HSL
Modern color syntax hsl() and hsla() allow mixed number and percentage components
Modern color syntax rgb() and rgba() allow mixed number and percentage components
Define the term "modern color syntax" (legacy color syntax already defined).
Consistently use the term "analogous components"
Changed to allow all predefined color spaces for interpolation
Clarified that for color(), three parameters (RGB or XYZ) are required
Clarified serialization of named colors, system colors, and transparent
Define specified value for Lab, LCH, Oklab, OkLCh
Define specified value for other sRGB colors
Define specified values for named and system colors
Clamp alpha, Lightness, Chroma and Hue at parsed-value time
Remove passing mention of specular white and CIE Lightness
No longer require as-specified Hue to be retained; clamp to [0, 360]
Consistent serialization of Lightness and number in examples
Minor typos and editorial clarifications
Changes since the
Candidate Recommendation of 5 July 2022
Removed hue interpolation "specified" value
Defined hue interpolation angle more precisely, maintaining differences of 360deg
Added example of carried forward alpha for premultiplication
Clarified a,b and C,h powerless at L=100% representing white.
Removed handwavy mention of L=400 which applies to hdr-CIELAB not CIE Lab
Consistent capitalization of Oklab and OkLCh
Moved definitions of valid color, invalid color, out of gamut and in gamut to terminology section
Fixed definition of "longer" hue interpolation
Further clarified the concept of a host syntax
Accessibility improvements for color swatches
Made explicit that legacy forms do not support "none"
Remove "none" from the hue production, as it is not allowed in legacy syntax
Removed some dangling references to CMYK and CMYKOGV, moved to CSS Color5
Clarified how missing values in colors to be interpolated are carried forward
Updated syntax of xyz-params so they take numbers and percentage, to align with prose
Ensure all examples and figures have IDs, self-links
Clarified importance to implementors of reading the gamut mapping introduction
Removed left-over mention of custom color spaces (feature was moved to CSS Color 5)
Refactor syntax of and
Editorial refactoring for better reading order.
Updated pseudocode for gamut mapping algorithm, remove un-needed deltaE calls
Changes since the
Working Draft of 28 June 2022
Updated status for Candidate Recommendation
Changes since the
Working Draft of 28 April 2022
Moved opacity property up to the top of the module, next to color property, before getting into details.
Improved description of the color property, in particular effect on other properties
Corrected longer hue adjust equation, for equal-modulo-360 colors
Added two new System colors: AccentColor and AccentColorText
Described overall color space conversion steps in new section
Accounted for
none
alpha in premultiplication and un-premultiplication
Changes since the
Working Draft of 15 December 2021
Made system colors fully resolve, but forbid their alteration in forced colors mode
Removed forgiveness for incorrect number of parameters in color() function
Changed serialization of CIE Lightness and OK Lightness to number rather than percentage.
Marked deprecated system color equivalences as at-risk
Added reference ranges to percentage values for CIE and OK L,a,b,C
Noted that there is sample code for performing and undoing premultiplication, for both rectangular and polar color spaces.
Added out of range clamping to the gamut mapping prose, as well as the pseudocode
Added normative reference for ProPhoto RGB / ROMM
Corrected sRGB and Display P3 black point value for reference surround
Added normative reference for Display P3
Avoided an infinite loop in gamut reduction, with colors whiter than white or darker than black
Clarified serialization of the
none
value
Clarified the opt-in to interpolation in Oklab, for non-legacy colors
Defined how premultiplication works, with the
none
value
Clarified that missing values in rgb serialize as 0
Clarified the use of calc() with the
none
value
Typo, inconsistent casing on System Colors
Added example of SelectedItem with SelectedItemText
Explicitly noted the presence or absence of legacy colors
Added normative reference for CIE XYZ
Added normative reference for HWB and HSL
Clarified that
hwb()
is not a legacy syntax so does not support the older, comma-separated syntactic form
Clarified that only legacy colors will gamut map, the others are unbounded
Use distinct terms, spectrophotometer and spectroradiometer
Assorted minor typos fixed, and grammatical improvements
Changes since the
Working Draft of 1 June 2021
Added gamut mapping section and defined a CSS gamut mapping algorithm as chroma reduction in OkLCh with local MINDE.
Computed value of color(xyz ...) is color(xyz-d65 ...)
Added srgb-linear to interpolation color spaces
Updated Changes from Colors 3 section
Added Resolving Oklab and OkLCh values section
Added srgb-linear color space
Moved @color-profile and device-cmyk to level 5 per CSSWG resolution
Defined interpolation color space
Clarified that matrices are row-major and linked to the matrix multiplication library
Split old Security & Privacy section into separate sections
Defined quirks-mode quirky hex colors
Removed fallback colors from device-cmyk
Host syntax that does not declare a default now uses Oklab by default
Added sample code for deltaE OK
Added sample conversion code for OKlab and OkLCh
Added oklab() and oklch() functions
Added description of Oklab and OkLCh
Added description of CIE LCH deficiencies
Allowed all components of a color to be "missing" via the
none
keyword,
defined when components are "powerless" and automatically become missing in some cases,
and fixed all references to "NaN" components to use the "missing" concept.
Defined explicit x,y whitepoint values, use consistently throughout
Defined the term host syntax
Defined context for resolving override-color colors
Added a new pair of system colors
Corrected HSL and HWB sample code
Replaced table of HSL values with error-free version
Added Lea Verou as co-editor by WG resolution
Clarified that hue angle is unbounded
MarkText example corrected
Added diagrams, corrected examples
Some editorial clarifications
Minor typos corrected, markup corrections
Changes since the
Working Draft of 12 November 2020
Noted indeterminate hue ssue on near-neutral Lab values converted to LCH
Clarified which steps are linear combinations in RGB Lab interconversion
Added components descriptor to @color-profile, for use in CSS Color 5
All predefined RGB color spaces are defined over the extended range
Clarified that there is no gamut mapping or gamut clipping step prior to color interpolation
Clarified interpolation of legacy sRGB syntaxes
Removed the lab option from
color()
List steps to interconvert between predefined color spaces
Consistent use of the term color space (two words)
Provided more guidance on selecting color space for mixing
Recalculated an example to increase precision
Added hue interpolation example
Simplified
color()
syntax by removing the fallback options
Clarified the types of ICC profile that may be linked from @color-profile
Support for the rare ICC Named Colors was removed
Improved precision of standard whitepoint chromaticities
Removed a trademark from description of one predefined color space
Rephrased interpolation to be more generic wrt to interpolation space
Corrected Accessibility Considerations section
Clarified that the color space argument for
color()
is mandatory, even for sRGB
Clarified that currentColor is not restricted to sRGB
Small correction to the sRGB to XYZ to sRGB matrices, improve round-tripping
Clarified the rec2020 transfer function, citing the correct ITU Rec BT.2020-2 reference
Correct fallback examples to use the correct syntax
Don’t force non-legacy colors to interpolate in a gamma-encoded space
Define premultiplied alpha interpolation
Start to address interpolation to and from currentColor
Define hue interpolation with NaN
Generalize color interpolation
Define interpolation to be in Lab, with override to LCG
Corrections to hue interpolation
Defined hue angle interpolation
Added interpolation section
Corrected syntax in some examples
Clarify exactly which components are allowed percentages, in
color()
Change to serialize
lch()
as itself rather than as
lab()
Minimum 10 bits per component precision for non-legacy sRGB in
color()
color space no longer optional in
color()
Consistent minimum precision between lab() and color(lab)
Clarified fallback procedure for the color() function – first valid in-gamut color, else first valid color which is then gamut mapped, else transparent black
Clarified difference between opacity property and colors with opacity, notably for rendering overlapping text glyphs
Added sample (but verified correct) code for ΔE2000
Added definition of previously-undefined term chromaticity, with examples; define chromaticity diagram.
Added explanation of color additivity, with examples
Added source links to WPT tests
Export definition of color, and valid color, for other specifications to reference
Define minimum number of bits per component, for serialization
Updated "applies to" definitions (CSS-wide change)
Added image state (display referred or scene referred) for predefined color spaces
Listed white point correlated color temperature (e.g. D65) alongside chromaticity coordinates, for clarity
Clarified that rounding is towards +∞
Correction of typos, markup corrections, link fixes
Changes since
Working Draft of
5 November 2019
Export some terms for use in other specifications
Update requirement from WCAG 2.0 to 2.1
Fully specify Unicode characters used for serialization
Define serialization of special named colors
Define serialization of device-cmyk()
Define serialization of
color()
Fully define RGB serialization, in maximally web-compatible way
Define serialization of Lab and LCH
Fully define serialization of alpha values
Consistency pass to avoid accidental RFC2119
Add IDs to all the examples, to enable referencing
Separate resolved color and serialized color sections
(Security) ICC profiles have no executable code
Define what out-of-range means for profiled colors
Clarify out-of-range clamping
Add examples of specified values
Clarify computed values
Resist fingerprinting, with mandatory mappings for deprecated system colors
Added explanatory note on history and reason for standardizing X11 colors
Correct hwb sample code
Add table of DeltaE2000 values for MacBeth patches
Add note on ICC profile Internet Media type
Add reference to PNG sRGB chunk
Clarify CMYK to Lab interconversion
Clarify RGB to Lab interconversion
More comparison of HSL vs. LCH
More description for Rec BT.2020 color space
Updated description of prophoto-rgb
Removed duplicate "keywords" from Value Definitions section
Added an example of an invalid color
Added example with multiple fallbacks
Assorted typos and markup fixes
Clarify handling for undeclared custom color spaces
Clarify some examples and explanatory notes
Handling for valid and invalid ICC profiles
Define handling for images with explicit tagged color space
Define color space for 4k, SDR video
State that user contrast settings mst take precedence
Clarify meaning of system colors outside for forced-color mode
Update default style rules
Add CIE XYZ color space to
color()
Greater clarity on hue angles, NaN explicitly allowed
Improve section on system color pairings, require AA accessible contrast
Warn of interaction between overlapping glyphs and the opacity property
Correct grammar in color definition
Improve description of Highlight/HighlightText
Correct prophoto-rgb transfer function
More precision for prophoto-rgb primaries
Started to define "can’t be displayed"
Removed paragraph about canvas surface
Added the buttonborder, mark, and marktext system colors
Added reverse conversion, sRGB to HWB
Clarified polar spaces are cylindrical, not spherical
Added an Accessibility Considerations section
Started to describe chroma-reduction gamut mapping rather than per-component clipping
Corrected white chromaticity for rec2020
Made device-cmyk available by @color-profile; updated CMYK to color algorithm to only use naive conversion as a last resort
Added print-oriented CMYK and KCMYOGV examples
User-defined color spaces now dashed-ident, making predefined color spaces extensible without clashes
Added lab option to the color() function
Added normative reference for CIE Lab
Clarified that prophoto-rgb uses
D50
whitepoint so does not require adaptation
Clarified direction of increasing angle in LCH
Clarified that color names are ASCII case insensitive
Initial value of the "color" property is now CanvasText
Removed confusing gray() function per CSS WG resolution
Collect scattered definitions into new
Color terminology
section
Add helpful figures and more examples
Minor editorial clarifications, spell check, fixing typos, bikeshed markup fixes
Changes since
Working Draft of 05 July 2016
Changed Lightness in Lab and LCH to be a percentage, for CSS compatibility
Clamping of color values clarified
Percentage opacity is now allowed
Define terms sRGB and linear-light sRGB, for use by other specs
Add new list of CSS system colors; renaming Text to CanvasText
Make system color keywords compute to themselves
Add computed/used entry for system colors
Rewrite intro to non-deprecated system colors to center their use around forced-colors mode rather than generic use
Consistent hyphenation of predefined color spaces
Restore text about non-opaque elements painting at layers even when not positioned
Initial value of the "color" property is now black
Clarify hue in LCH is modulo 360deg (change now reverted)
Clarify allowed range of L in LCH and Lab, and meaning of L=100
Update references for color spaces used in video
Add prophoto-rgb predefined color space
Correct black and white luminance levels for display-p3
Clarify display-p3 transfer function
Add a98-rgb color space, correct table of primary chromaticities
Clarify that currentColor’s computed value is not the resolved color
Update syntax is examples to conform to latest specification
Remove the color-mod() function
Drop the "media" from propdef tables
Export, and consistently use, "transparent black" and "opaque black"
Clarify calculated values such as percents
Clarify required precision and rounding behavior for color components
Clarify "color" property has no effect on color font glyphs (unless specifically referenced, e.g. with currentColor)
Clarify how color values are resolved
Clarify that HSL, HWB and named colors resolve to sRGB
Simplify conversion from device-cmyk to sRGB
Describe previous, comma-using color syntaxes as "legacy"; change examples to commaless form
Remove superfluous requirement that displayed colors be restricted to device gamut (like there was any other option!)
Rename P3 to display-p3; avoid claiming this is DCI P3, as these are not the same
Improved description of the parameters to the "color()" function
Disallow predefined spaces from "@color-profile" identifier
Add canonical order to "color", "color-adjust" and "opacity" property definitions
Switch definition of alpha compositing from SVG11 to CSS Compositing
Clarify sample conversion code is non-normative
Add Security and Privacy Considerations
Update several references to most current versions
Convert inline issues to links to GitHub issues
Minor editorial clarifications, formatting and markup improvements
Changes from Colors 3
The primary change, compared to CSS Color 3,
is that CSS colors are no longer restricted to the
narrow gamut of sRGB.
To support this, several brand new features have been added:
predefined, wide color gamut RGB color spaces
lab()
lch()
oklab()
and
oklch()
functions, for device-independent color
Other technical changes:
Serialization of

is now specified here, rather than in the CSS Object Model
hwb()
function, for specifying sRGB colors in the HWB notation.
Addition of named color
rebeccapurple
In addition, there have been some syntactic changes:
rgb()
and
rgba()
functions now accept

rather than

hsl()
and
hsla()
functions now accept

as well as

for hues.
rgb()
and
rgba()
, and
hsl()
and
hsla()
are now aliases of each other
(all of them have an optional alpha).
rgb()
rgba()
hsl()
, and
hsla()
have all gained a new syntax
consisting of space-separated arguments
and an optional slash-separated opacity.
All the color functions use this syntax form now,
in keeping with
CSS’s functional-notation design principles
All uses of

now accept

as well as

4 and 8-digit hex colors have been added, to specify transparency.
The
none
value has been added, to represent powerless components.
21.
Security Considerations
The system colors,
if they actually correspond to the user’s system colors,
pose a security risk,
as they make it easier for a malware site
to create user interfaces that appear to be from the system.
However, as several system colors are now defined to be "generic",
this risk is believed to be mitigated.
22.
Privacy Considerations
This specification defines "system" colors,
which theoretically can expose details of the user’s OS settings,
which is a fingerprinting risk.
23.
Accessibility Considerations
This specification
encourages authors to not use
color alone
as a distinguishing feature.
This specification
encourages browsers to ensure
adequate contrast for specific system color foreground/background pairs
A harder requirement with specific AA or AAA contrast ratios was considered,
but since browsers are often just passing along color choices made by the OS,
or selected by users (who may have particular requirements,
including lower contrast for people living with migraines or epileptic seizures), the CSSWG
was unable to require a specific contrast level.
Conformance
Document conventions
Conformance requirements are expressed with a combination of
descriptive assertions and RFC 2119 terminology. The key words “MUST”,
“MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
“RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
document are to be interpreted as described in RFC 2119.
However, for readability, these words do not appear in all uppercase
letters in this specification.
All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes.
[RFC2119]
Examples in this specification are introduced with the words “for example”
or are set apart from the normative text with
class=
"example"
like this:
This is an example of an informative example.
Informative notes begin with the word “Note” and are set apart from the
normative text with
class=
"note"
, like this:
Note, this is an informative note.
Advisements are normative sections styled to evoke special attention and are
set apart from other normative text with
"advisement"
, like
this:
UAs MUST provide an accessible alternative.
Tests
Tests relating to the content of this specification
may be documented in “Tests” blocks like this one.
Any such block is non-normative.
Conformance classes
Conformance to this specification
is defined for three conformance classes:
style sheet
CSS
style sheet
renderer
UA
that interprets the semantics of a style sheet and renders
documents that use them.
authoring tool
UA
that writes a style sheet.
A style sheet is conformant to this specification
if all of its statements that use syntax defined in this module are valid
according to the generic CSS grammar and the individual grammars of each
feature defined in this module.
A renderer is conformant to this specification
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the features defined
by this specification by parsing them correctly
and rendering the document accordingly. However, the inability of a
UA to correctly render a document due to limitations of the device
does not make the UA non-conformant. (For example, a UA is not
required to render color on a monochrome monitor.)
An authoring tool is conformant to this specification
if it writes style sheets that are syntactically correct according to the
generic CSS grammar and the individual grammars of each feature in
this module, and meet all other conformance requirements of style sheets
as described in this module.
Partial implementations
So that authors can exploit the forward-compatible parsing rules to
assign fallback values, CSS renderers
must
treat as invalid (and
ignore
as appropriate
) any at-rules, properties, property values, keywords,
and other syntactic constructs for which they have no usable level of
support. In particular, user agents
must not
selectively
ignore unsupported component values and honor supported values in a single
multi-value property declaration: if any value is considered invalid
(as unsupported values must be), CSS requires that the entire declaration
be ignored.
Implementations of Unstable and Proprietary Features
To avoid clashes with future stable CSS features,
the CSSWG recommends
following best practices
for the implementation of
unstable
features and
proprietary extensions
to CSS.
Non-experimental implementations
Once a specification reaches the Candidate Recommendation stage,
non-experimental implementations are possible, and implementors should
release an unprefixed implementation of any CR-level feature they
can demonstrate to be correctly implemented according to spec.
To establish and maintain the interoperability of CSS across
implementations, the CSS Working Group requests that non-experimental
CSS renderers submit an implementation report (and, if necessary, the
testcases used for that implementation report) to the W3C before
releasing an unprefixed implementation of any CSS features. Testcases
submitted to W3C are subject to review and correction by the CSS
Working Group.
Further information on submitting testcases and implementation reports
can be found from on the CSS Working Group’s website at
Questions should be directed to the
public-css-testsuite@w3.org
mailing list.
CR exit criteria
For this specification to be advanced to Proposed Recommendation,
there must be at least two independent, interoperable implementations
of each feature. Each feature may be implemented by a different set of
products, there is no requirement that all features be implemented by
a single product. For the purposes of this criterion, we define the
following terms:
independent
each implementation must be developed by a
different party and cannot share, reuse, or derive from code
used by another qualifying implementation. Sections of code that
have no bearing on the implementation of this specification are
exempt from this requirement.
interoperable
passing the respective test case(s) in the
official CSS test suite, or, if the implementation is not a Web
browser, an equivalent test. Every relevant test in the test
suite should have an equivalent test created if such a user
agent (UA) is to be used to claim interoperability. In addition
if such a UA is to be used to claim interoperability, then there
must one or more additional UAs which can also pass those
equivalent tests in the same way for the purpose of
interoperability. The equivalent tests must be made publicly
available for the purposes of peer review.
implementation
a user agent which:
implements the specification.
is available to the general public. The implementation may
be a shipping product or other publicly available version
(i.e., beta version, preview release, or "nightly build").
Non-shipping product releases must have implemented the
feature(s) for a period of at least one month in order to
demonstrate stability.
is not experimental (i.e., a version specifically designed
to pass the test suite and is not intended for normal usage
going forward).
The specification will remain Candidate Recommendation for at least
six months.
Index
Terms defined by this specification
a98-rgb
, in § 10.6
absolute color
, in § 4.1
AccentColor
, in § 6.2
AccentColorText
, in § 6.2
ActiveBorder
, in § Unnumbered section
ActiveCaption
, in § Unnumbered section
ActiveText
, in § 6.2
additive color space
, in § 2
aliceblue
, in § 6.1
alpha channel
, in § 4
alpha component
, in § 4

, in § 4.2
analogous components
, in § 13.2
analogous set
, in § 13.2
antiquewhite
, in § 6.1
AppWorkspace
, in § Unnumbered section
aqua
, in § 6.1
aquamarine
, in § 6.1
azure
, in § 6.1
Background
, in § Unnumbered section
beige
, in § 6.1
Binary Search Gamut Map with Local MINDE
, in § 14.2.2
bisque
, in § 6.1
black
, in § 6.1
blanchedalmond
, in § 6.1
blue
, in § 6.1
blueviolet
, in § 6.1
brown
, in § 6.1
burlywood
, in § 6.1
ButtonBorder
, in § 6.2
ButtonFace
, in § 6.2
ButtonHighlight
, in § Unnumbered section
ButtonShadow
, in § Unnumbered section
ButtonText
, in § 6.2
cadetblue
, in § 6.1
calibrated
, in § 2
can be displayed
, in § 10.1
can’t be displayed
, in § 10.1
Canvas
, in § 6.2
CanvasText
, in § 6.2
CaptionText
, in § Unnumbered section
carried forward
, in § 13
cast a ray
, in § 14.2.6
characterized
, in § 2
chartreuse
, in § 6.1
chocolate
, in § 6.1
chromatic adaptation transform
, in § 9.1
chromaticity
, in § 2

, in § 4.1
color
(property)
, in § 3.2
definition of
, in § 2
color()
, in § 10.1

, in § 4.1

, in § 4.1
color functions
, in § 4

, in § 13.1

, in § 13.1
color space
, in § 2

, in § 10.1
computed color
, in § 15
convert a color
, in § 11
coral
, in § 6.1
cornflowerblue
, in § 6.1
cornsilk
, in § 6.1
crimson
, in § 6.1
css gamut mapped
, in § 14.2
CSS gamut mapping algorithms
, in § 14.2
currentcolor
, in § 6.4
cyan
, in § 6.1
cylindrical polar color
, in § 4
D50
, in § 2
D65
, in § 2
darkblue
, in § 6.1
darkcyan
, in § 6.1
darkgoldenrod
, in § 6.1
darkgray
, in § 6.1
darkgreen
, in § 6.1
darkgrey
, in § 6.1
darkkhaki
, in § 6.1
darkmagenta
, in § 6.1
darkolivegreen
, in § 6.1
darkorange
, in § 6.1
darkorchid
, in § 6.1
darkred
, in § 6.1
darksalmon
, in § 6.1
darkseagreen
, in § 6.1
darkslateblue
, in § 6.1
darkslategray
, in § 6.1
darkslategrey
, in § 6.1
darkturquoise
, in § 6.1
darkviolet
, in § 6.1
decreasing
, in § 13.4.4
deeppink
, in § 6.1
deepskyblue
, in § 6.1

, in § Unnumbered section
dimgray
, in § 6.1
dimgrey
, in § 6.1
display-p3
, in § 10.4
display-p3-linear
, in § 10.5
dodgerblue
, in § 6.1
equivalent colors
, in § 12
Field
, in § 6.2
FieldText
, in § 6.2
firebrick
, in § 6.1
floralwhite
, in § 6.1
forestgreen
, in § 6.1
fuchsia
, in § 6.1
gainsboro
, in § 6.1
gamut
, in § 2
ghostwhite
, in § 6.1
gold
, in § 6.1
goldenrod
, in § 6.1
gray
, in § 6.1
GrayText
, in § 6.2
green
, in § 6.1
greenyellow
, in § 6.1
grey
, in § 6.1

, in § 5.2
hex color
, in § 5.2
hex color notation
, in § 5.2
Highlight
, in § 6.2
HighlightText
, in § 6.2
honeydew
, in § 6.1
host syntax
, in § 13.1
hotpink
, in § 6.1
HSL
, in § 7
hsl()
, in § 7
hsla()
, in § 7
HTML-compatible serialization is requested
, in § 16.2.1

, in § 4.3

, in § 13.1
HWB
, in § 8
hwb()
, in § 8
InactiveBorder
, in § Unnumbered section
InactiveCaption
, in § Unnumbered section
InactiveCaptionText
, in § Unnumbered section
increasing
, in § 13.4.3
indianred
, in § 6.1
indigo
, in § 6.1
InfoBackground
, in § Unnumbered section
InfoText
, in § Unnumbered section
in-gamut
, in § 2
interpolation color space
, in § 13
invalid color
, in § 2
ivory
, in § 6.1
khaki
, in § 6.1
Lab
, in § 9.3
lab()
, in § 9.3
lavender
, in § 6.1
lavenderblush
, in § 6.1
lawngreen
, in § 6.1
LCH
, in § 9.3
lch()
, in § 9.3
legacy color syntax
, in § 4.1.2

, in § 7

, in § 7

, in § 5.1

, in § 5.1
lemonchiffon
, in § 6.1
lightblue
, in § 6.1
lightcoral
, in § 6.1
lightcyan
, in § 6.1
lightgoldenrodyellow
, in § 6.1
lightgray
, in § 6.1
lightgreen
, in § 6.1
lightgrey
, in § 6.1
lightpink
, in § 6.1
lightsalmon
, in § 6.1
lightseagreen
, in § 6.1
lightskyblue
, in § 6.1
lightslategray
, in § 6.1
lightslategrey
, in § 6.1
lightsteelblue
, in § 6.1
lightyellow
, in § 6.1
lime
, in § 6.1
limegreen
, in § 6.1
linen
, in § 6.1
LinkText
, in § 6.2
longer
, in § 13.4.2
luminance
, in § 2
magenta
, in § 6.1
Mark
, in § 6.2
MarkText
, in § 6.2
maroon
, in § 6.1
mediumaquamarine
, in § 6.1
mediumblue
, in § 6.1
mediumorchid
, in § 6.1
mediumpurple
, in § 6.1
mediumseagreen
, in § 6.1
mediumslateblue
, in § 6.1
mediumspringgreen
, in § 6.1
mediumturquoise
, in § 6.1
mediumvioletred
, in § 6.1
, in § Unnumbered section
MenuText
, in § Unnumbered section
midnightblue
, in § 6.1
MINDE
, in § 14.1.2
mintcream
, in § 6.1
missing
, in § 4.4
missing color component
, in § 4.4
missing component
, in § 4.4
mistyrose
, in § 6.1
moccasin
, in § 6.1
modern color syntax
, in § 4.1.1

, in § 7

, in § 7

, in § 5.1

, in § 5.1

, in § 6.1
named color
, in § 6.1
navajowhite
, in § 6.1
navy
, in § 6.1
none
, in § 4.4
Oklab
, in § 9.4
oklab()
, in § 9.4
OkLCh
, in § 9.4
oklch()
, in § 9.4
oldlace
, in § 6.1
olive
, in § 6.1
olivedrab
, in § 6.1
opacity
, in § 3.3

, in § 3.3
opaque black
, in § 4
orange
, in § 6.1
orangered
, in § 6.1
orchid
, in § 6.1
out of gamut
, in § 2
palegoldenrod
, in § 6.1
palegreen
, in § 6.1
paleturquoise
, in § 6.1
palevioletred
, in § 6.1
papayawhip
, in § 6.1
parse a CSS value
, in § 4.5
peachpuff
, in § 6.1
peru
, in § 6.1
pink
, in § 6.1
plum
, in § 6.1

, in § 13.1
powderblue
, in § 6.1
powerless
, in § 4.4.1
powerless color component
, in § 4.4.1
powerless component
, in § 4.4.1

, in § 10.1

, in § 10.1
premultiplied
, in § 13.3
prepare a color col1 for conversion
, in § 11
prophoto-rgb
, in § 10.7
purple
, in § 6.1

, in § Unnumbered section
Ray Trace Gamut Map
, in § 14.2.6
rebeccapurple
, in § 6.1
rec2020
, in § 10.8

, in § 13.1
rectangular orthogonal color
, in § 4
red
, in § 6.1
resolve to sRGB
, in § 4.1
rgb()
, in § 5.1
rgba()
, in § 5.1
rosybrown
, in § 6.1
royalblue
, in § 6.1
saddlebrown
, in § 6.1
salmon
, in § 6.1
sandybrown
, in § 6.1
Scrollbar
, in § Unnumbered section
seagreen
, in § 6.1
seashell
, in § 6.1
SelectedItem
, in § 6.2
SelectedItemText
, in § 6.2
shorter
, in § 13.4.1
sienna
, in § 6.1
silver
, in § 6.1
skyblue
, in § 6.1
slateblue
, in § 6.1
slategray
, in § 6.1
slategrey
, in § 6.1
snow
, in § 6.1
springgreen
, in § 6.1
sRGB
, in § 10.2
srgb
, in § 10.2
sRGB-linear
, in § 10.3
srgb-linear
, in § 10.3
steelblue
, in § 6.1
support legacy color syntax
, in § 4.1

, in § 6.2
system color pairings
, in § 6.2
system colors
, in § 6.1
tagged image
, in § 3.4
tan
, in § 6.1
teal
, in § 6.1
thistle
, in § 6.1
ThreeDDarkShadow
, in § Unnumbered section
ThreeDFace
, in § Unnumbered section
ThreeDHighlight
, in § Unnumbered section
ThreeDLightShadow
, in § Unnumbered section
ThreeDShadow
, in § Unnumbered section
tomato
, in § 6.1
transparent
, in § 6.3
transparent black
, in § 4
turquoise
, in § 6.1
untagged image
, in § 3.5
untagged video
, in § 3.5
used color
, in § 15
valid color
, in § 2
violet
, in § 6.1
VisitedText
, in § 6.2
wheat
, in § 6.1
white
, in § 6.1
white point
, in § 2
whitesmoke
, in § 6.1
Window
, in § Unnumbered section
WindowFrame
, in § Unnumbered section
WindowText
, in § Unnumbered section
xyz
, in § 10.9
xyz-d50
, in § 10.9
xyz-d65
, in § 10.9

, in § 10.1

, in § 10.1
yellow
, in § 6.1
yellowgreen
, in § 6.1
Terms defined by reference
[CSS-ANIMATIONS-1]
defines the following terms:
animation
[CSS-BACKGROUNDS-3]
defines the following terms:
background
background-color
border-bottom-color
border-color
border-left-color
border-right-color
border-top-color
[CSS-CASCADE-5]
defines the following terms:
computed value
declared value
inherited value
initial value
specified value
used value
[CSS-COLOR-5]
defines the following terms:
color-mix()
relative color
[CSS-COLOR-ADJUST-1]
defines the following terms:
forced colors mode
used color scheme
[CSS-CONDITIONAL-3]
defines the following terms:
@supports
supports(conditionText)
[CSS-IMAGES-4]
defines the following terms:

[CSS-SYNTAX-3]
defines the following terms:





parse
[CSS-TEXT-DECOR-4]
defines the following terms:
text-emphasis-color
[CSS-TRANSITIONS-1]
defines the following terms:
transition
[CSS-UI-4]
defines the following terms:
accent color
accent-color
[CSS-VALUES-4]
defines the following terms:
-infinity





calc()
canonical unit
CSS-wide keywords
functional notation
implementation-defined limit for values approaching infinity
infinity
keyword
NaN
{A}
[CSS2]
defines the following terms:
auto
stacking context
z-index
[CSSOM-1]
defines the following terms:
resolved value
[DOM]
defines the following terms:
element
quirks mode
[FILTER-EFFECTS-1]
defines the following terms:
filter
[HTML]
defines the following terms:
mark
[INFRA]
defines the following terms:
string
[MEDIAQUERIES-5]
defines the following terms:
media feature
References
Normative References
[Bradford-CAT]
Ming R. Luo; R. W. G. Hunt.
A Chromatic Adaptation Transform and a Colour Inconstancy Index. Color Research & Application 23(3) 154-158
. June 1998.
[CIELAB]
ISO/CIE 11664-4:2019(E): Colorimetry — Part 4: CIE 1976 L*a*b* colour space
. 2019. Published. URL:
[COLORIMETRY]
Colorimetry, Fourth Edition. CIE 015:2018
. 2018. URL:
[Compositing]
Chris Harrelson.
Compositing and Blending Level 1
. 21 March 2024. CRD. URL:
[CSS-BACKGROUNDS-3]
Elika Etemad; Brad Kemper.
CSS Backgrounds and Borders Module Level 3
. 11 March 2024. CRD. URL:
[CSS-CASCADE-5]
Elika Etemad; Miriam Suzanne; Tab Atkins Jr..
CSS Cascading and Inheritance Level 5
. 13 January 2022. CR. URL:
[CSS-COLOR-ADJUST-1]
Elika Etemad; et al.
CSS Color Adjustment Module Level 1
. 16 December 2025. CR. URL:
[CSS-CONDITIONAL-3]
Chris Lilley; David Baron; Elika Etemad.
CSS Conditional Rules Module Level 3
. 15 August 2024. CRD. URL:
[CSS-SYNTAX-3]
Tab Atkins Jr.; Simon Sapin.
CSS Syntax Module Level 3
. 24 December 2021. CRD. URL:
[CSS-TEXT-DECOR-4]
Elika Etemad; Koji Ishii.
CSS Text Decoration Module Level 4
. 4 May 2022. WD. URL:
[CSS-UI-4]
Tab Atkins Jr.; Florian Rivoal.
CSS Basic User Interface Module Level 4
. 20 January 2026. WD. URL:
[CSS-VALUES-3]
Tab Atkins Jr.; Elika Etemad.
CSS Values and Units Module Level 3
. 22 March 2024. CRD. URL:
[CSS-VALUES-4]
Tab Atkins Jr.; Elika Etemad.
CSS Values and Units Module Level 4
. 12 March 2024. WD. URL:
[CSS2]
Bert Bos; et al.
Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification
. 7 June 2011. REC. URL:
[CSSOM-1]
Daniel Glazman; Emilio Cobos Álvarez.
CSS Object Model (CSSOM)
. 26 August 2021. WD. URL:
[Display-P3]
Apple, Inc.
Display P3
. 2022-02. URL:
[DOM]
Anne van Kesteren.
DOM Standard
. Living Standard. URL:
[HSL]
George H. Joblove, Donald Greenberg.
Color spaces for computer graphics
. August 1978. URL:
[HTML]
Anne van Kesteren; et al.
HTML Standard
. Living Standard. URL:
[HWB]
Alvy Ray Smith.
HWB — A More Intuitive Hue-Based Color Model
. 1996. URL:
[ICC]
ICC.1:2022 (Profile version 4.4.0.0)
. May 2022. URL:
[INFRA]
Anne van Kesteren; Domenic Denicola.
Infra Standard
. Living Standard. URL:
[ITU-R-BT.601]
Recommendation ITU-R BT.601
. URL:
[ITU-R-BT.709]
Recommendation ITU-R BT.709
. URL:
[MEDIAQUERIES-5]
Tab Atkins Jr.; et al.
Media Queries Level 5
. 19 February 2026. WD. URL:
[Oklab]
Björn Ottosson.
A perceptual color space for image processing
. December 2020. URL:
[Rec.2020]
Recommendation ITU-R BT.2020-2: Parameter values for ultra-high definition television systems for production and international programme exchange
. October 2015. URL:
[RFC2119]
S. Bradner.
Key words for use in RFCs to Indicate Requirement Levels
. March 1997. Best Current Practice. URL:
[ROMM]
ISO 22028-2:2013 Photography and graphic technology — Extended colour encodings for digital image storage, manipulation and interchange — Part 2: Reference output medium metric RGB colour image encoding (ROMM RGB)
. 2013-04. URL:
[SMPTE296]
ST 296:2012, 1280 × 720 Progressive Image 4:2:2 and 4:4:4 Sample Structure — Analog and Digital Representation and Analog Interface
. 17 May 2012. Standard. URL:
[SRGB]
Multimedia systems and equipment - Colour measurement and management - Part 2-1: Colour management - Default RGB colour space - sRGB
. URL:
[SVG11]
Erik Dahlström; et al.
Scalable Vector Graphics (SVG) 1.1 (Second Edition)
. 16 August 2011. REC. URL:
Informative References
[Coloraide-MINDE]
Isaac Muse.
OkLCh Chroma
. 2024. URL:
[Coloraide-Ray-Trace]
Isaac Muse.
Ray Tracing Chroma Reduction
. 2024. URL:
[colorjs-EdgeSeeker]
Alexey Ardov.
EdgeSeeker Gamut Mapping algorithm
. 2023. URL:
[colorjs-MINDE]
Chris Lilley.
Gamut mapping
. 2022. URL:
[colorjs-RayTrace]
Isaac Muse.
Ray Trace Gamut Mapping
. 2023. URL:
[CSS-ANIMATIONS-1]
David Baron; et al.
CSS Animations Level 1
. 2 March 2023. WD. URL:
[CSS-COLOR-5]
Chris Lilley; Una Kravets; Lea Verou.
CSS Color Module Level 5
. 13 April 2026. WD. URL:
[CSS-CONDITIONAL-5]
Chris Lilley; et al.
CSS Conditional Rules Module Level 5
. 30 October 2025. WD. URL:
[CSS-IMAGES-4]
Elika Etemad; Tab Atkins Jr.; Lea Verou.
CSS Images Module Level 4
. 30 September 2025. WD. URL:
[CSS-TRANSITIONS-1]
Chris Marrin; et al.
CSS Transitions Module Level 1
. 8 January 2026. WD. URL:
[CSS3-TEXT-DECOR]
Elika Etemad; Koji Ishii.
CSS Text Decoration Module Level 3
. 5 May 2022. CRD. URL:
[DCI-P3]
SMPTE Recommended Practice - D-Cinema Quality — Reference Projector and Environment
. 2011. URL:
[FILTER-EFFECTS-1]
Dirk Schulze; Dean Jackson.
Filter Effects Module Level 1
. 18 December 2018. WD. URL:
[JPEG]
Eric Hamilton.
JPEG File Interchange Format
. September 1992. URL:
[PNG]
Chris Lilley; et al.
Portable Network Graphics (PNG) Specification (Third Edition)
. 24 June 2025. REC. URL:
[REC_BT.1886]
ITU-R BT.1886 Reference electro-optical transfer function for flat panel displays used in HDTV studio production
. 2011-03. URL:
[ROMM-RGB]
ROMM RGB
. URL:
[Sharma]
G. Sharma; W. Wu; E. N. Dalal.
The CIEDE2000 Color-Difference Formula: Implementation Notes, Supplementary Test Data, and Mathematical Observations
. February 2005. URL:
[TIFF]
TIFF Revision 6.0
. 3 June 1992. URL:
[Understanding_CCT]
What is CCT? A Guide to Choosing Correlated Color Temperature for Your Lighting
. 2024-08-14. URL:
[WCAG21]
Michael Cooper; et al.
Web Content Accessibility Guidelines (WCAG) 2.1
. 6 May 2025. REC. URL:
[Wolfe]
Geoff Wolfe.
Design and Optimization of the ProPhoto RGB Color Encodings
. 2011-12-21. URL:
Property Index
Name
Value
Initial
Applies to
Inh.
%ages
Anim­ation type
Canonical order
Com­puted value
color

CanvasText
all elements and text
yes
N/A
by computed value type
per grammar
computed color, see resolving color values
opacity

all elements
no
map to the range [0,1]
by computed value type
per grammar
specified number, clamped to the range [0,1]
Issues Index
add pseudocode for EdgeSeeker GMA