CSS Values and Units Module Level 4

CSS Values and Units Module Level 4
CSS Values and Units Module Level 4
Editor’s Draft
16 April 2026
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Tab Atkins
Google
Elika J. Etemad / fantasai
Apple
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Abstract
This CSS module describes the common values and units that CSS properties accept and the syntax used for describing them in CSS property definitions.
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 is a public copy of the editors’ draft.
It is provided for discussion only and may change at any moment.
Its publication here does not imply endorsement of its contents by W3C.
Don’t cite this document other than as work in progress.
Please send feedback
by
filing issues in GitHub
(preferred),
including the spec code “css-values” in the title, like this:
“[css-values]
…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
1.
Introduction
The value definition field of each CSS property can contain keywords,
data types (which appear between
and
),
and information on how they can be combined.
Generic data types (

being the most widely used)
that can be used by many properties are described in this specification,
while more specific data types (e.g.,

are described in the corresponding modules.
1.1.
Module Interactions
This module replaces and extends the data type definitions in
[CSS2]
sections
1.4.2.1
4.3
and
A.2
2.
Value Definition Syntax
The
value definition syntax
described here
is used to define the set of valid values for CSS properties
(and the valid syntax of many other parts of CSS).
A value so described can have one or more components.
2.1.
Component Value Types
Component value types are designated in several ways:
Keyword
values (such as
auto
disc
, etc.)
and at-keywords representing the start of an
at-rule
which appear literally, without quotes (e.g.
auto
or
@media
).
Note:
It is possible, with
escaping
, to construct a
CSS identifier
whose value ends with
or starts with
Such a token is an

(i.e. a
keyword
),
not a

or an

Basic data types,
which appear between
and
(e.g.,


, etc.).
For
numeric data types
this type notation can annotate any range restrictions
using the
bracketed range notation
described below.
Property value ranges,
which represent the same pattern of values as a property bearing the same name.
These are written as the property name,
surrounded by single quotes,
between
and
e.g.,
<'border-width'>
<'background-attachment'>
, etc.
These types
do not
include
CSS-wide keywords
such as
inherit
Additionally,
if the property’s value grammar
is a
comma-separated repetition
the corresponding type
does not include the top-level
comma-separated list multiplier
(E.g. if a property named
pairing
is defined as


then
'pairing'
is equivalent to


not


.)
Why remove the multiplier?
The top-level multiplier is ripped out of these value types
because top-level comma-separated repetitions are mostly used for
coordinating list properties
and when a shorthand combines several such properties,
it needs the unmultiplied grammar
so it can construct its
own
comma-separated repetition.
Without this special treatment,
every such longhand would have to be defined
with an ad-hoc production just for the inner value,
which makes the grammars harder to understand overall.
Functional notations and their arguments.
These may be written literally as defined in
§ 2.6 Functional Notation Definitions
or referenced by a non-terminal using the function’s name,
followed by an empty parentheses pair,
between
and
e.g.

and references the correspondingly-named
functional notation
Other non-terminals.
These are written as the name of the non-terminal
between
and
as in

Notice the distinction between

and
<'border-width'>
the latter represents the grammar of the
border-width
property,
the former requires an explicit expansion elsewhere.
The definition of a non-terminal is typically located near its first appearance in the specification.
Delimiters, which represent their corresponding tokens.
Slashes (
),
commas
),
colons (
),
semicolons (
),
parentheses (
and
),
and braces (
and
are written literally.
Other delimiters must be written enclosed in single quotes
(such as
'+'
).
Commas
specified in the grammar are implicitly omissible
in some circumstances,
when used to separate optional terms in the grammar.
Within a top-level list in a property or other CSS value,
or a function’s argument list,
a comma specified in the grammar must be omitted if:
all items preceding the comma have been omitted
all items following the comma have been omitted
multiple commas would be adjacent (ignoring
white space
/comments),
due to the items between the commas being omitted.
For example, if a function can accept three arguments in order,
but all of them are optional,
the grammar can be written like:
example
first
second
third
Given this grammar,
writing
example(first, second, third)
is valid,
as is
example(first, second)
or
example(first, third)
or
example(second)
However,
example(first, , third)
is invalid, as one of those commas are no longer separating two options;
similarly,
example(,second)
and
example(first,)
are invalid.
example(first second)
is also invalid,
as commas are still required to actually separate the options.
If commas were not implicitly omittable,
the grammar would have to be much more complicated
to properly express the ways that the arguments can be omitted,
greatly obscuring the simplicity of the feature.
All CSS properties also accept the
CSS-wide keyword values
as the sole component of their property value.
For readability these are not listed explicitly in the property value syntax definitions.
For example, the full value definition of
border-color
under
CSS Cascading and Inheritance Level 3
is

| inherit | initial | unset
(even though it is listed as

).
Note:
This implies that, in general,
combining these keywords with other component values in the same declaration
results in an invalid declaration.
For example,
background: url(corner.png) no-repeat, inherit;
is invalid.
2.2.
Component Value Combinators
Component values can be arranged into property values as follows:
Juxtaposing components means that
all of them must occur, in the given order.
A double ampersand (
&&
) separates two or more components,
all of which must occur, in any order.
A double bar (
||
) separates two or more options:
one or more of them must occur, in any order.
A bar (
) separates two or more alternatives:
exactly one of them must occur.
Brackets ([ ]) are for grouping.
Juxtaposition is stronger than the double ampersand, the double
ampersand is stronger than the double bar, and the double bar
is stronger than the bar. Thus, the following lines are equivalent:
a b | c || d && e f
a b
c ||
d &&
e f
]]]
For reorderable combinators (||, &&),
ordering of the grammar does not matter:
components in the same grouping may be interleaved in any order.
Thus, the following lines are equivalent:
a || b || c
b || a || c
Note:
Combinators are
not
associative, so grouping is significant.
For example,
a || b || c
and
a || [ b || c ]
are distinct grammars:
the first allows a value like
b a c
, but the second does not.
2.3.
Component Value Multipliers
Every type, keyword, or bracketed group may be followed by one of
the following modifiers:
An asterisk (
indicates that the preceding type, word, or group
occurs zero or more times.
A plus (
indicates that the preceding type, word, or group
occurs one or more times.
A question mark (
indicates that the preceding type, word, or group
is optional (occurs zero or one times).
A single number in curly braces (
indicates that the preceding type, word, or group
occurs
times.
A comma-separated pair of numbers in curly braces (
indicates that the preceding type, word, or group
occurs at least
and at most
times.
The
may be omitted ({
,})
to indicate that there must be at least
repetitions,
with no upper bound on the number of repetitions.
A hash mark (
indicates that the preceding type, word, or group
occurs one or more times, separated by comma tokens
(which may optionally be surrounded by
white space
and/or comments).
It may optionally be followed by the curly brace forms, above,
to indicate precisely how many times the repetition occurs,
like
#{1,4}
An exclamation point (
) after a group
indicates that the group is required
and must produce at least one value;
even if the grammar of the items within the group
would otherwise allow the entire contents to be omitted,
at least one component value must not be omitted.
The
and
multipliers may be stacked as
+#
similarly, the
and
multipliers,
{A}
and
multipliers, and
{A,B}
and
multipliers may be stacked as
#?
{A}?
, and
{A,B}?
, respectively.
These stacks each represent the later multiplier
applied to the result of the earlier multiplier.
(These same stacks can be represented using grouping,
but in complex grammars this can push the number of brackets beyond readability.)
For repeated component values (indicated by
, or
),
UAs
must support at least 20 repetitions of the component.
If a property value contains more than the supported number of repetitions,
the declaration must be ignored as if it were invalid.
2.4.
Combinator and Multiplier Patterns
There are a small set of common ways to combine multiple independent
component values
in particular numbers and orders.
In particular, it’s common to want to express that,
from a set of component value,
the author must select zero or more, one or more, or all of them,
and in either the order specified in the grammar or in any order.
All of these can be easily expressed using simple patterns of
combinators
and
multipliers
in order
any order
zero or more
A? B? C?
A? || B? || C?
one or more
A? B? C?
A || B || C
all
A B C
A && B && C
Note that all of the "any order" possibilities are expressed using combinators,
while the "in order" possibilities are all variants on juxtaposition.
2.5.
Component Values and White Space
Unless otherwise specified,
white space
and/or comments may appear before, after, and/or between
components combined using the above
combinators
and
multipliers
Note:
In many cases, spaces will in fact be
required
between components
in order to distinguish them from each other.
For example, the value
1em2em
would be parsed as a single

with the number
and the identifier
em2em
which is an invalid unit.
In this case, a space would be required before the
to get this parsed as the two lengths
1em
and
2em
2.6.
Functional Notation Definitions
The syntax of a
functional notation
is defined
as a sequence of:
The function’s name written as an identifier
followed by an open parenthesis
(such as
example(
),
or the

production
to indicate a function with an arbitrary name.
The function’s arguments, if any,
expressed using the
value definition syntax
A literal closing parenthesis.
The function’s arguments are considered
implicitly grouped
as if surrounded by brackets (
[ ... ]
).
For example, a grammar like:
example


will match a function whose name is "example"
and whose arguments match "


".
For example, the Selectors grammar defines pseudo-classes generically,
allowing any possibly function name after the initial colon:
= :
:
This represents
any
function name,
with

as the function arguments.
Since the
functional notation
implicitly groups
its contents,
the effect of any combinator inside it
is scoped to the function’s argument.
For example, the
functional notation
syntax definition
example( foo | bar )
is equivalent to
example( [ foo | bar ] )
2.7.
Property Value Examples
Below are some examples of properties with their corresponding value
definition fields
Property
Value definition field
Example value
orphans

text-align
left | right | center | justify
center
padding-top
|
5%
outline-color
| invert
#fefefe
text-decoration
none | underline || overline || line-through || blink
overline underline
font-family
[ | ]#
"Gill Sans", Futura, sans-serif
border-width
[ | thick | medium | thin ]{1,4}
2px medium 4px
box-shadow
[ inset? && {2,4} && ? ]# | none
3px 3px rgba(50%, 50%, 50%, 50%), lemonchiffon 0 0 4px inset
2.8.
Non-Terminal Definitions and Grammar Production Blocks
The precise grammar of non-terminals, like

or

is often specified in a
CSS grammar production block
These are conventionally represented in a preformatted block of definitions
like this:
The

syntax is defined as follows:
= keyword

some-really-long-pattern-of-stuff
=
Each definition starts on its own line,
and consists of the non-terminal to be defined,
followed by an
followed by the fragment of
value definition syntax
to which it expands.
A definition can stretch across multiple lines,
and terminates before the next line that starts a new grammar production
or at the end of the grammar production block
(whichever comes first).
In the above example, the

definition covers two lines.
The third line starts a new definition for

(A naked
is never valid in
value definition syntax
so it’s unambiguous when a new line starts a fresh definition.)
3.
Combining Values: Interpolation, Addition, and Accumulation
Some procedures, for example
transitions
and
animations
combine
two CSS property values.
The following combining operations—​on the two
computed values
and
yielding the
computed value
result
—​are defined.
For operations that are not commutative
(for example, matrix multiplication,
or accumulation of mismatched transform lists)
represents
the first term of the operation and
represents
the second.
interpolation
Given two property values
and
produces an intermediate value
result
at a distance of
along the interval between
and
such that
= 0 produces
and
= 1 produces
The range of
is (−∞, ∞)
due to the effect of
timing functions
As a result, this procedure must also define
extrapolation behavior for
outside [0, 1].
addition
Given two property values
and
returns the sum of the two properties,
result
Note:
While
addition
can often be expressed
in terms of the same weighted sum function
used to define
interpolation
this is not always the case.
For example, interpolation of transform matrices involves
decomposing and interpolating the matrix components
whilst addition relies on matrix multiplication.
If a value type does not define a specific procedure for
addition
or is defined as
not additive
its
addition
operation is simply
result
accumulation
Given two property values
and
returns the result,
result
of combining the two operands
such that
is treated as a
delta
from
Note: For many types of animation such as numbers or lengths,
accumulation
is defined to be identical
to
addition
A common case where the definitions differ
is for list-based types
where
addition
may be defined as appending to a list
whilst
accumulation
may be defined
as component-based addition.
For example, the filter list values
blur(2)
and
blur(3)
when
added
together would produce
blur(2) blur(3)
but when
accumulated
would produce
blur(5)
If a value type does not define a specific procedure for
accumulation
its
accumulation
operation is identical to
addition
These operations are only defined on
computed values
(As a result, it is not necessary to define, for example,
how to add a

value of
15pt
with
5em
since such values will be resolved to their
canonical unit
before being passed to any of the above procedures.)
3.1.
Range Checking
Interpolation can result in a value outside the valid range for a property,
even if all of the inputs to interpolation are valid;
this especially happens when
is outside the [0, 1] range,
but some
easing functions
can cause this to occur even within that range.
If the final result
after
interpolation, addition, and accumulation
is out-of-range for the target context the value is being used in,
it does not cause the declaration to be invalid.
Instead, the value must be clamped to the range allowed in the target context,
exactly the same as
math functions
(see
§ 10.12 Range Checking
).
Note:
Even if interpolation results in an out-of-range value,
addition/accumulation might "correct" the result and bring it back into range.
Thus, clamping is only applied to the
final
result
of applying all interpolation-related operations.
4.
Textual Data Types
The
textual data types
include
various keywords and identifiers
as well as strings (

) and URLs (

).
Aside from the casing of
pre-defined keywords
or as explicitly defined for a given property,
no normalization is performed,
not even Unicode normalization:
the
specified
and
computed value
of a property
are exactly the provided Unicode values after parsing
(which includes character set conversion and
escaping
).
[UNICODE]
[CSS-SYNTAX-3]
CSS
identifiers
generically denoted by

consist of a sequence of characters conforming to the

grammar.
[CSS-SYNTAX-3]
Identifiers cannot be quoted;
otherwise they would be interpreted as strings.
CSS properties accept two classes of
identifiers
pre-defined keywords
and
author-defined identifiers
Note:
The

production is not meant for property value definitions—​

should be used instead.
It is provided as a convenience for defining other syntactic constructs.
All textual data types
interpolate
as
discrete
and are
not additive
4.1.
Pre-defined Keywords
In the value definition fields,
keywords
with a pre-defined meaning appear literally.
Keywords are
identifiers
and are interpreted
ASCII case-insensitively
(i.e., [a-z] and [A-Z] are equivalent).
For example, here is the value definition for the
border-collapse
property:
Value
collapse | separate
And here is an example of its use:
table
border-collapse
separate
4.1.1.
CSS-wide keywords:
initial
inherit
and
unset
As defined
above
all properties accept the
CSS-wide keywords
which represent value computations common to all CSS properties.
These keywords are normatively defined in
the
CSS Cascading and Inheritance Module
Tests
multicol-inherit-002.xht
(live test)
(source)
multicol-rule-color-inherit-001.xht
(live test)
(source)
multicol-rule-color-inherit-002.xht
(live test)
(source)
units-008.xht
(live test)
(source)
Other CSS specifications can define additional CSS-wide keywords.
4.2.
Unprefixed Author-defined Identifiers: the

type
Some properties accept arbitrary author-defined identifiers as a component value.
This generic data type is denoted by

and represents any valid
CSS identifier
that would not be misinterpreted as a pre-defined keyword in that property’s value definition.
Such identifiers are fully case-sensitive
(meaning they’re compared using the "
identical to
" operation),
even in the ASCII range
(e.g.
example
and
EXAMPLE
are two different, unrelated user-defined identifiers).
The
CSS-wide keywords
are not valid

s.
The
default
keyword is reserved
and is also not a valid

Specifications using

must specify clearly
what other keywords are excluded from

, if any—​for example by saying that any pre-defined keywords in that property’s value definition are excluded.
Excluded keywords are excluded in all
ASCII case permutations
When parsing positionally-ambiguous keywords in a property value,

production can only claim the keyword if no other unfulfilled production can claim it.
For example, the shorthand declaration
animation: ease-in ease-out
is equivalent to the longhand declarations
animation-timing-function: ease-in; animation-name: ease-out;
ease-in
is claimed by the

production belonging to
animation-timing-function
leaving
ease-out
to be claimed by the

production belonging to
animation-name
Note:
When designing grammars with

the

should always be “positionally unambiguous”,
so that it’s impossible to conflict with any keyword values in the property.
Such conflicts can alternatively be avoided by using

4.3.
Prefixed Author-defined Identifiers: the

type
Some contexts accept
both
author-defined identifiers
and
CSS-defined identifiers.
If not handled carefully,
this can result in difficulties adding new CSS-defined values;
UAs
have to study existing usage
and gamble that there are sufficiently few author-defined identifiers in use
matching the new CSS-defined one,
so giving the new value a special CSS-defined meaning
won’t break existing pages.
While there are many legacy cases in CSS
that mix these two values spaces in exactly this fraught way,
the

type is meant to be an easy way
to distinguish author-defined identifiers
from CSS-defined identifiers.
The

production
is a

with all the case-sensitivity that implies,
with the additional restriction that it must start with two dashes
(U+002D HYPHEN-MINUS).

s are reserved solely for use as author-defined names.
CSS will never define a

for its own use.
For example,
custom properties
need to be distinguishable from CSS-defined properties,
as new properties are added to CSS regularly.
To allow this,
custom property
names are required to be

s,
as in this example:
.foo
--fg-color
blue

s are also used in the
@color-profile
rule,
to separate author-defined color profiles
from pre-defined ones like
device-cmyk
and allow CSS to define more pre-defined (but overridable) profiles in the future
without fear of clashing with author-defined profiles:
@color-profile
--foo
src
url
);
.foo
color
color
--foo
.5
.2
);
CSS will use

more in the future,
as more author-controlled syntax is added.
CSS authoring tools,
such as preprocessors that turn custom syntax into standard CSS,
should
use

as well,
to avoid clashing with future CSS design.
For example,
if a CSS preprocessor added a new "custom" at-rule,
it
shouldn’t
spell it
@custom
as this would clash with a future official
@custom
rule added by CSS.
Instead, it should use
@--custom
which is guaranteed to never clash with anything defined by CSS.
Even better, it should use
@--library1-custom
so that if Library2 adds their own "custom" at-rule
(spelled @--library2-custom),
there’s no possibility of clash.
Ideally this prefix should be customizable,
if allowed by the tooling,
so authors can manually avoid clashes on their own.
4.4.
Quoted Strings: the

type
Strings
are denoted by

When written literally,
they consist of a sequence of characters delimited by double quotes or single quotes,
corresponding to the

production
in the
CSS Syntax Module
[CSS-SYNTAX-3]
Double quotes cannot occur inside double quotes, unless
escaped
(as
"\"
or as
"\22"
).
Analogously for single quotes (
'\'
or
'\27'
).
content
"this is a 'string'."
content
"this is a \"
string\
"."
content
'this is a "string".'
content
'this is a \'
string\
'.'
It is possible to break strings over several lines, for aesthetic or
other reasons, but in such a case the newline itself has to be escaped
with a backslash (\). The newline is subsequently removed from the
string. For instance, the following two selectors are exactly the
same:
Example(s):
title=
"a not s\
o very long title"
/*...*/
title=
"a not so very long title"
/*...*/
Since a string cannot directly represent a newline, to include a
newline in a string, use the escape "\A". (Hexadecimal A is the line
feed character in Unicode (U+000A), but represents the generic notion
of "newline" in CSS.)
4.5.
Resource Locators: the

type
The

type,
written with the
url()
and
src()
functions,
represents a
URL
which is a pointer to a resource.
The syntax of

is:

url
()
src
()
url
()
> =
url



src
()
> =
src


This example shows a URL being used as a background image:
body
background
url
"http://www.example.com/pinkish.gif"
url()
can be written without quotation marks around the URL value,
in which case it is
specially-parsed
as a

; see
CSS Syntax 3
§ 4.3.6 Consume a url token
[CSS-SYNTAX-3]
Note:
Because of this special parsing,
url()
can only express its value literally.
To provide a URL by functions such as
var()
use the
src()
notation,
which does not have this special parsing rule.
For example, the following declarations are identical:
background
url
"http://www.example.com/pinkish.gif"
);
background
url
);
And these have the same meaning as well:
background
src
"http://www.example.com/pinkish.gif"
);
--foo
"http://www.example.com/pinkish.gif"
background
src
var
--foo
));
But this does
not
work:
--foo
"http://www.example.com/pinkish.gif"
background
url
var(--foo
));
...because the unescaped "(" in the value causes a parse error,
so the entire declaration is thrown out as invalid.
Note:
The unquoted
url()
syntax cannot accept a

argument
and has extra escaping requirements:
parentheses,
whitespace
characters,
single quotes (') and double quotes (") appearing in a URL
must be escaped with a backslash,
e.g.
url(open\(parens)
url(close\)parens)
(In quoted

url()
s,
only newlines and the character used to quote the string need to be escaped.)
Depending on the type of URL,
it might also be possible to write these characters as URL-escapes
(e.g.
url(open%28parens)
or
url(close%29parens)
as described in
[URL]
Some CSS contexts (such as
@import
) also allow a

to be represented by a bare

, without the function wrapper.
In such cases the string behaves identically to a
url()
function containing that string.
For example, the following statements act identically:
@import
url
"base-theme.css"
);
@import
"base-theme.css"
4.5.1.
Relative URLs
In order to create modular style sheets that are not dependent on
the absolute location of a resource, authors should use relative URLs.
Relative URLs (as defined in
[URL]
) are resolved to full URLs
using a base URL. RFC 3986, section 3, defines the normative
algorithm for this process.
For CSS style sheets, the base URL is that of the style sheet itself,
not that of the styled source document.
Style sheets embedded within a document have
the base URL associated with their container.
Note:
For HTML documents,
the
base URL is mutable
When a

appears in the computed value of a property,
it is
resolved to an absolute URL
The computed value of a URL that the
UA
cannot resolve to an absolute URL is the specified value.
For example, suppose the following rule:
body
background
url
"tile.png"
is located in a style sheet designated by the URL:
http
//www.example.org/style/basic.css
The background of the source document’s

will be tiled with whatever image is described by the resource designated by the URL:
http
//www.example.org/style/tile.png
The same image will be used regardless of the URL of the source document containing the

4.5.1.1.
Fragment URLs
To enable element ID references to work in CSS
regardless of base URL changes
or shadow DOM,

s have special behavior
when they contain only a fragment.
If a

’s value starts with a U+0023 NUMBER SIGN (
) character,
then the URL additionally has its
local url flag
set,
and is a
tree-scoped reference
for the URL’s
fragment
When matching a

with the
local url flag
set:
if the URL’s fragment is an element ID reference
(rather than, say, a media fragment),
resolve it as a
tree-scoped reference
with the tree’s IDs as the associated
tree-scoped names
specifically, resolve to the first element
in
tree order
among the associated
node tree
’s descendants
with the URL’s
fragment
as its ID.
(And, as usual for
tree-scoped references
continuing up to the host’s tree if needed.)
If no such element is found,
the URL fails to resolve.
otherwise, resolve the fragment against the current document.
Possibly reference
find a potential indicated element
but that is defined specifically for
Document
s,
not
ShadowRoot
s.
Note:
This means that such fragments will resolve
against the contents of the current document
(or whichever
node tree
the stylesheet lives in,
if shadow DOM is involved)
regardless of how such relative URLs would resolve elsewhere
(ignoring, for example,
base
elements
changing the base URL,
or relative URLs in linked stylesheets
resolving against the stylesheet’s URL).
In the following example,
#anchor
will resolve against
http
//example.com/
whereas
#image
will resolve against the elements in the HTML document itself:

base
href
"http://example.com/"
...
href
"#anchor"
style
"background-image: url(#image)"
link
When
serializing
url()
with the
local url flag
set,
it must serialize as just the fragment.
4.5.2.
Empty URLs
If the value of the

is the empty string
(like
url("")
or
url()
),
the url must resolve to an invalid resource
(similar to what the url
about:invalid
does).
Its computed value is
url("")
or
src("")
whichever was specified,
and it must serialize as such.
Tests
empty.html
(live test)
(source)
Note:
This matches the behavior of empty urls for embedded resources elsewhere in the web platform,
and avoids excess traffic re-requesting the stylesheet or host document
due to editing mistakes leaving the
url()
value empty,
which are almost certain to be invalid resources for whatever the
url()
shows up in.
Linking on the web platform
does
allow empty urls,
so if/when CSS gains some functionality to control hyperlinks,
this restriction can be relaxed in those contexts.
4.5.3.
URL Modifiers

s support specifying additional

s,
which change the meaning or the interpretation of the URL somehow.

is either an

or a
functional notation
This specification does not define any

s,
but other specs may do so.
Note:

that is either unquoted or not wrapped in
url()
notation
cannot accept any

s.
4.5.4.
URL Processing Model
To compute the
style resource base URL
for a
CSS rule
or a
CSS declaration block
cssRuleOrDeclaration
Let
sheet
be null.
If
cssRuleOrDeclaration
is a
CSS declaration block
whose
parent CSS rule
is not null,
set
cssRuleOrDeclaration
to
cssRuleOrDeclaration
’s
parent CSS rule
If
cssRuleOrDeclaration
is a
CSS rule
, set
sheet
to
cssRuleOrDeclaration
’s
parent style sheet
If
sheet
is not null:
If
sheet
’s
stylesheet base URL
is not null,
return
sheet
’s
stylesheet base URL
If
sheet
’s
location
is not null, return
sheet
’s
location
Return
cssRuleOrDeclaration
’s
relevant settings object
’s
API base URL
To
resolve a style resource URL
from a
url
or

urlValue
and a
CSS rule
or a
CSS declaration block
cssRuleOrDeclaration
Let
base
be the
style resource base URL
given
cssRuleOrDeclaration
Return the result of the
URL parser
steps with
urlValue
’s
url
and
base
To
fetch a style resource
from a
url
or

urlValue
given an
CSS rule
or a
css declaration block
cssRuleOrDeclaration
a string
destination
matching a
RequestDestination
a "no-cors" or "cors"
corsMode
and an algorithm
processResponse
accepting a
response
and a null, failure or byte stream:
Let
parsedUrl
be the result of
resolving
urlValue
given
cssRuleOrDeclaration
If that failed, return.
Let
settingsObject
be
cssRuleOrDeclaration
’s
relevant settings object
Let
req
be a new
request
whose
url
is
parsedUrl
whose
destination
is
destination
mode
is
corsMode
origin
is
settingsObject
’s
origin
credentials mode
is "same-origin",
use-url-credentials flag
is set,
client
is
settingsObject
and whose
referrer
is "client".
If
corsMode
is "no-cors", set
req
’s
credentials mode
to "include".
Apply any
URL request modifier steps
that apply to this request.
Note:
This specification does not define any URL request modification steps,
but other specs may do so.
If
req
’s
mode
is "cors", and
sheet
is not null, then
set
req
’s
referrer
to the
style resource base URL
given
cssRuleOrDeclaration
[CSSOM]
If
sheet
’s
origin-clean flag
is set,
set
req
’s
initiator type
to "css".
[CSSOM]
Fetch
req
with
processResponseConsumeBody
set to
processResponse
When interpreting
URLs
expressed in CSS,
the
URL parser’s
encoding
argument
must be omitted
(i.e. use the default, UTF-8),
regardless of the stylesheet encoding.
Note:
In other words, a URL written in CSS
will always
percent-encode
non-ASCII codepoints
using UTF-8 in the
URL
object
(and thus whenever using the
URL
value for e.g. network requests),
regardless of the stylesheet’s own encoding.
Note that this occurs
after decoding the stylesheet
into Unicode
code points
5.
Numeric Data Types
Numeric data types are used to represent
quantities, indexes, positions, and other such values.
Although many syntactic variations can exist
in expressing the quantity (numeric aspect) in a given numeric value,
the
specified
and
computed value
do not distinguish these variations:
they represent the value’s abstract quantity,
not its syntactic representation.
The
numeric data types
include



and various
dimensions
including




, and

Note:
While general-purpose
dimensions
are defined here,
some other modules define additional data types
(e.g.
[css-grid-1]
introduces
fr
units)
whose usage is more localized.
The precision and supported range of numeric values in CSS
is
implementation-defined
and can vary based on the property or other context a value is used in.
However, within the CSS specifications,
infinite precision and range is assumed.
When a value cannot be explicitly supported due to range/precision limitations,
it must be converted to the closest value supported by the implementation,
but how the implementation defines "closest" is
implementation-defined
as well.
If an

must be converted
due to exceeding the implementation-defined range of supported values,
it must be clamped to the nearest supported multiple of
360deg
5.1.
Range Restrictions and Range Definition Notation
Properties can restrict numeric values to some range.
If the value is outside the allowed range,
then unless otherwise specified,
the declaration is invalid and must be
ignored
Range restrictions can be annotated in the numeric type notation
using
CSS bracketed range notation
—​
min
max
—​within the angle brackets, after the identifying keyword,
indicating a closed range
between (and including)
min
and
max
For example,

indicates an integer between
and
10
, inclusive,
while

indicates an angle between
0deg
and
180deg
(expressed in any unit).
Note:
CSS values generally do not allow open ranges;
thus only square-bracket notation is used.
CSS theoretically supports infinite precision and infinite ranges for all value types;
however in reality implementations have finite capacity.
UAs
should support reasonably useful ranges and precisions.
Range extremes that are ideally unlimited
are indicated using ∞ or −∞ as appropriate.
For example,

indicates a non-negative length.
If no range is indicated,
either by using the
bracketed range notation
or in the property description,
then
−∞
is assumed.
Values of −∞ or ∞ must be written without units,
even if the value type uses units.
Values of
can
be written without units,
even if the value type doesn’t allow “unitless zeroes”
(such as

).
Note:
At the time of writing,
the
bracketed range notation
is new;
thus in most CSS specifications
any range limitations are described only in prose.
(For example, “Negative values are not allowed” or
“Negative values are invalid”
indicate a
range.)
This does not make them any less binding.
5.2.
Integers: the

type
Integer values are denoted by

When written literally,
an
integer
is one or more decimal digits
through
and corresponds to a subset of the

production
in the CSS Syntax Module
[CSS-SYNTAX-3]
The first digit of an integer may be immediately preceded by
or
to indicate the integer’s sign.
Tests
multicol-count-non-integer-001.xht
(live test)
(source)
multicol-count-non-integer-002.xht
(live test)
(source)
multicol-count-non-integer-003.xht
(live test)
(source)
numbers-units-001.xht
(live test)
(source)
numbers-units-004.xht
(live test)
(source)
Unless otherwise specified,
in the CSS specifications
rounding to the nearest integer
requires rounding in the direction of +∞
when the fractional portion is exactly 0.5.
(For example,
1.5
rounds to
, while
-1.5
rounds to
-1
.)
5.2.1.
Computation and Combination of

Unless otherwise specified,
the
computed value
of a specified

is the specified abstract integer.
Interpolation
of

is defined as
result
round((1 -
) ×
);
that is, interpolation happens in the real number space
as for

s, and the result is converted to an

by
rounding to the nearest integer
Addition
of

is defined as
result
Tests
calc-positive-fraction-001.html
(live test)
(source)
rgba-011.html
(live test)
(source)
5.3.
Real Numbers: the

type
Number values are denoted by

and represent real numbers, possibly with a fractional component.
Tests
animation-iteration-count-calc.html
(live test)
(source)
numbers-units-002.xht (visual test)
(source)
numbers-units-003.xht
(live test)
(source)
When written literally,
number
is either an
integer
or zero or more decimal digits followed by a dot (.) followed by one or more decimal digits;
optionally, it can be concluded by the letter “e” or “E”
followed by an integer indicating the base-ten exponent
in
scientific notation
It corresponds to the

production
in the
CSS Syntax Module
[CSS-SYNTAX-3]
As with integers, the first character of a number
may be immediately preceded by
or
to indicate the number’s sign.
The value

represents a literal
number
with the value 0.
Expressions that merely evaluate to a

with the value 0
(for example,
calc(0)
do not match

only literal

s do.
5.3.1.
Computation and Combination of

Unless otherwise specified,
the
computed value
of a specified

is the specified abstract number.
Interpolation
of

is defined as
result
(1 -
) ×
Addition
of

is defined as
result
5.4.
Numbers with Units:
dimension
values
The general term
dimension
refers to
a number with a unit attached to it;
and is denoted by

When written literally,
dimension
is a
number
immediately followed by a unit identifier,
which is an
identifier
It corresponds to the

production
in the
CSS Syntax Module
[CSS-SYNTAX-3]
Like keywords, unit identifiers are
ASCII case-insensitive
Tests
angle-units-003.html
(live test)
(source)
CSS uses

s to specify
distances (

),
durations (

),
frequencies (

),
resolutions (

),
and other quantities.
5.4.1.
Compatible Units
When
serializing
computed values
[CSSOM]
compatible units
(those related by a static multiplicative factor,
like the 96:1 factor between
px
and
in
or the computed
font-size
factor between
em
and
px
are converted into a single
canonical unit
Each group of compatible units defines which among them is the
canonical unit
that will be used for serialization.
When serializing
resolved values
that are
used values
all value types (percentages, numbers, keywords, etc.)
that represent lengths are considered
compatible
with lengths.
Likewise any future API that returns
used values
must consider any values that represent distances/durations/frequencies/etc.
as
compatible
with the relevant class of
dimensions
and canonicalize accordingly.
Tests
calc-serialization-002.html
(live test)
(source)
5.4.2.
Combination of Dimensions
Interpolation
of
compatible
dimensions
(for example, two

values)
is defined as
result
(1 -
) ×
Addition
of
compatible
dimensions
is defined as
result
5.5.
Percentages: the

type
Percentage values are denoted by

and indicates a value that is some fraction of another reference value.
When written literally,
percentage
consists of a
number
immediately followed by a percent sign
It corresponds to the

production
in the
CSS Syntax Module
[CSS-SYNTAX-3]
Percentage values are always relative to another quantity,
for example a length.
Each property that allows percentages also defines the quantity to which the percentage refers.
This quantity can be a value of another property for the same element,
the value of a property for an ancestor element,
a measurement of the formatting context
(e.g., the width of a
containing block
),
or something else.
Tests
numbers-units-016.xht
(live test)
(source)
numbers-units-017.xht
(live test)
(source)
5.5.1.
Computation and Combination of

Unless otherwise specified
(such as in
font-size
, which computes its

values to

),
the
computed value
of a percentage
is the specified percentage.
Interpolation
of

is defined as
result
(1 -
) ×
Addition
of

is defined as
result
5.6.
Mixing Percentages and Dimensions
In cases where a

can represent the same quantity
as a
dimension
in the same
component value
position,
and can therefore be combined with them in a
calc()
expression,
the following convenience notations may be used in the property grammar:

Equivalent to


where the

will resolve to a


Equivalent to


where the

will resolve to a


Equivalent to


where the

will resolve to an


Equivalent to


where the

will resolve to a

For example, the
width
property can accept a

or a

both representing a measure of distance.
This means that
width: calc(500px + 50%);
is allowed—​both values are converted to absolute lengths and added.
If the containing block is
1000px
wide,
then
width: 50%;
is equivalent to
width: 500px
and
width: calc(50% + 500px)
thus ends up equivalent to
width: calc(500px + 500px)
or
width: 1000px
On the other hand, the second and third arguments of the
hsl()
function
can only be expressed as

s.
Although
calc()
productions are allowed in their place,
they can only combine percentages with themselves,
as in
calc(10% + 20%)
Note:
Specifications should never alternate

in place of a dimension
in a grammar unless they are
compatible
Note:
More <
type
-percentage> productions can be added in the future as needed.
A will never be added,
as

and

can’t be combined in
calc()
5.6.1.
Computation and Combination of Percentage and Dimension Mixes
The
computed value
of a percentage-dimension mix is defined as
a computed dimension if the percentage component is zero
or is defined specifically to compute to a dimension value
a computed percentage if the dimension component is zero
computed calc() expression
otherwise
Interpolation
of percentage-dimension value combinations
(e.g.




or equivalent notations)
is defined as
equivalent to
interpolation
of

if both
and
are pure

values
equivalent to
interpolation
of

if both
and
are pure

values
equivalent to converting both values into a
calc()
expression
representing the sum of the dimension type and a percentage
(each possibly zero)
and
interpolating
each component individually
(as a




and as a

, respectively)
Addition
of

is defined
the same as
interpolation
except by
adding
each component
rather than
interpolating
it.
5.7.
Ratios: the

type
Ratio values are denoted by

and represent the ratio of two numeric values.
It most often represents an aspect ratio,
relating a width (first) to a height (second).
When written literally,
ratio
has the syntax:

The second

is optional,
defaulting to
However,

is always serialized with both components.
The computed value of a

is the pair of numbers provided.
If either number in the

is 0 or infinite,
it represents a
degenerate ratio
(and, generally, won’t do anything).
If two

s need to be compared,
divide the first number by the second,
and compare the results.
For example,
3/2
is less than
2/1
because it resolves to 1.5 while the second resolves to 2.
(In other words, “tall” aspect ratios are less than “wide” aspect ratios.)
5.7.1.
Combination of

The interpolation of a

is defined
by converting each

to a number
by dividing the first value by the second
(so a ratio of
3 / 2
would become
1.5
),
taking the logarithm of that result
(so the
1.5
would become approximately
0.176
),
then interpolating those values.
The result during the interpolation
is converted back to a

by inverting the logarithm,
then interpreting the result as a

with the result as the first value and
as the second value.
If either

is
degenerate
the values cannot be interpolated.
For example,
halfway through a linear interpolation from
5 / 1
to
3 / 2
the result is approximately the ratio
2.73 / 1
(roughly
11 / 4
, slightly taller than a
3 / 1
ratio):
start =
log
);
// ≈
0.69897
end =
log
1.5
);
// ≈
0.17609
interp =
0.69897
.5
0.17609
.5
// ≈
0.43753
final =
10
^interp
// ≈
2.73
Note:
Interpolating over the logarithm of the ratio
means the results are scale-independent
5 / 1
to
300 / 200
would give the same results as above),
that they’re symmetrical over "wide" and "tall" variants
(interpolating from
1 / 5
to
2 / 3
would give
a ratio approximately equal to
1 / 2.73
at the halfway point),
and that they’re symmetrical over whether the width is fixed and the height is based on the ratio
or vice versa.
These properties are not shared by many other possible interpolation strategies.
Note:
Due to the properties of logarithms,
any log can be used;
the example here uses base-10 log,
but if, say, the natural log and e was used,
the intermediate results would be different
but the final result would be the same.
Addition of

s is not possible.
6.
Distance Units: the

type
Lengths refer to distance measurements
and are denoted by

in the property definitions.
A length is a
dimension
For zero lengths the unit identifier is optional
(i.e. can be syntactically represented as the

).
However, if a
could be parsed as either a

or a

in a property
(such as
line-height
),
it must parse as a

Properties may restrict the length value to some range.
If the value is outside the allowed range,
the declaration is invalid and must be
ignored
Tests
min-width-001.xht
(live test)
(source)
numbers-units-005.xht
(live test)
(source)
numbers-units-006.xht
(live test)
(source)
While some properties allow negative length values,
this may complicate the formatting and there may be implementation-specific limits.
If a negative length value is allowed but cannot be supported,
it must be converted to the nearest value that can be supported.
In cases where the
used
length cannot be supported,
user agents must approximate it in the
actual
value.
There are two types of length units:
relative
and
absolute
The
specified value
of a length (
specified length
) is
represented by its quantity and its unit.
The
computed value
of a length (
computed length
) is
the
specified length
resolved to an
absolute length
and its unit is not distinguished:
it can be represented by any
absolute length unit
(but will be serialized using its
canonical unit
px
).
Tests
calc-unit-analysis.html
(live test)
(source)
shape-outside-circle-002.html
(live test)
(source)
shape-outside-circle-004.html
(live test)
(source)
shape-outside-ellipse-002.html
(live test)
(source)
shape-outside-ellipse-004.html
(live test)
(source)
shape-outside-inset-003.html
(live test)
(source)
shape-outside-polygon-004.html
(live test)
(source)
While the exact supported precision of numeric values,
and how they are rounded to match that precision,
is generally
implementation-defined

s in
border-width
and a few other properties
are rounded in a specific fashion
to ensure reasonable visual display.
(This algorithm is called by individual properties explicitly.)
To
snap a length as a border width
given a

len
Assert:
len
is non-negative.
If
len
is an integer number of
device pixels
do nothing.
If
len
is greater than zero, but less than 1
device pixel
round
len
up to 1
device pixel
If
len
is greater than 1
device pixel
round it down to the nearest integer number of
device pixels
6.1.
Relative Lengths
Relative length units
specify a length relative to another length.
Style sheets that use relative units can more easily scale from one output environment to another.
The relative units are:
Informative Summary of Relative Units
unit
relative to
em
font size of the element
ex
x-height of the element’s font
cap
cap height (the nominal height of capital letters) of the element’s font
ch
typical
character advance
of a narrow glyph in the element’s font,
as represented by the “0” (ZERO, U+0030) glyph
ic
typical
character advance
of a fullwidth glyph in the element’s font,
as represented by the “水” (CJK water ideograph, U+6C34) glyph
rem
font size of the root element
lh
line height of the element
rlh
line height of the root element
vw
1% of viewport’s width
vh
1% of viewport’s height
vi
1% of viewport’s size in the root element’s
inline axis
vb
1% of viewport’s size in the root element’s
block axis
vmin
1% of viewport’s smaller dimension
vmax
1% of viewport’s larger dimension
Child elements do not inherit the relative values as specified for their parent;
they inherit the
computed values
6.1.1.
Font-relative Lengths: the
em
rem
ex
rex
cap
rcap
ch
rch
ic
ric
lh
rlh
units
The
font-relative lengths
refer to the font metrics
either of the element on which they are used
(for the
local font-relative lengths
or of the root element
(for the
root font-relative lengths
).
Common typographic metrics
em
Equal to the computed value of the
font-size
property of the element on which it is used.
The rule:
h1
line-height
1.2
em
means that the line height of
h1
elements
will be 20% greater than the font size of
h1
element.
On the other hand:
h1
font-size
1.2
em
means that the font size of
h1
elements
will be 20% greater than the computed font size inherited by
h1
elements.
rem
Equal to the computed value of the
em
unit on the root element.
Tests
calc-rem-lang.html
(live test)
(source)
ex
Equal to the used x-height of the
first available font
[CSS3-FONTS]
The x-height is so called because it is often equal to the height of the lowercase "x".
However, an
ex
is defined even for fonts that do not contain an "x".
The x-height of a font can be found in different ways.
Some fonts contain reliable metrics for the x-height.
If reliable font metrics are not available,
UAs
may determine the x-height from the height of a lowercase glyph.
One possible heuristic is to look at
how far the glyph for the lowercase "o" extends below the baseline,
and subtract that value from the top of its bounding box.
In the cases where it is impossible or impractical to determine the x-height,
a value of 0.5em must be assumed.
Tests
ex-unit-001.html
(live test)
(source)
ex-unit-002.html
(live test)
(source)
ex-unit-003.html
(live test)
(source)
numbers-units-007.xht
(live test)
(source)
numbers-units-009.xht
(live test)
(source)
numbers-units-010.xht
(live test)
(source)
numbers-units-011.xht
(live test)
(source)
numbers-units-012.xht
(live test)
(source)
numbers-units-013.xht
(live test)
(source)
numbers-units-015.xht
(live test)
(source)
numbers-units-019.xht
(live test)
(source)
units-001.xht
(live test)
(source)
units-002.xht
(live test)
(source)
units-003.xht
(live test)
(source)
units-004.xht
(live test)
(source)
units-005.xht
(live test)
(source)
calc-ch-ex-lang.html
(live test)
(source)
rex
Equal to the value of the
ex
unit on the root element.
cap
Equal to the used cap-height of the
first available font
[CSS3-FONTS]
The cap-height is so called because it is approximately equal to the height of a capital Latin letter.
However, a
cap
is defined even for fonts that do not contain Latin letters.
The cap-height of a font can be found in different ways.
Some fonts contain reliable metrics for the cap-height.
If reliable font metrics are not available,
UAs
may determine the cap-height from the height of an uppercase glyph.
One possible heuristic is to look at
how far the glyph for the uppercase “O” extends below the baseline,
and subtract that value from the top of its bounding box.
In the cases where it is impossible or impractical to determine the cap-height,
the font’s ascent must be used.
rcap
Equal to the value of the
cap
unit on the root element.
ch
Represents the typical
advance measure
of European alphanumeric characters,
and measured as the used
advance measure
of the “0” (ZERO, U+0030) glyph
in the font used to render it.
(The
advance measure
of a glyph is its advance width or height,
whichever is in the inline axis of the element.)
Note:
This measurement is an approximation
(and in monospace fonts, an exact measure)
of a single narrow glyph’s
advance measure
thus allowing measurements based on an expected glyph count.
Note:
The advance measure of a glyph depends on writing-mode and text-orientation
as well as font settings, text-transform, and any other properties that affect glyph selection or orientation.
In the cases where it is impossible or impractical to determine the measure of the “0” glyph,
it must be assumed to be 0.5em wide by 1em tall.
Thus, the
ch
unit falls back to
0.5em
in the general case,
and to
1em
when it would be typeset upright
(i.e.
writing-mode
is
vertical-rl
or
vertical-lr
and
text-orientation
is
upright
).
Tests
ch-unit-001.html
(live test)
(source)
ch-unit-002.html
(live test)
(source)
ch-unit-003.html
(live test)
(source)
ch-unit-004.html
(live test)
(source)
ch-unit-008.html
(live test)
(source)
ch-unit-009.html
(live test)
(source)
ch-unit-010.html
(live test)
(source)
ch-unit-011.html
(live test)
(source)
ch-unit-012.html
(live test)
(source)
ch-unit-016.html
(live test)
(source)
ch-unit-017.html
(live test)
(source)
line-break-ch-unit.html
(live test)
(source)
calc-ch-ex-lang.html
(live test)
(source)
rch
Equal to the value of the
ch
unit on the root element.
ic
Represents the typical
advance measure
of CJK letters,
and measured as the used
advance measure
of the “水” (CJK water ideograph, U+6C34) glyph
found in the font used to render it.
Note:
This measurement is a typically an exact measure
(in the few fonts with proportional fullwidth glyphs, an approximation)
of a single
fullwidth
glyph’s
advance measure
thus allowing measurements based on an expected glyph count.
In the cases where it is impossible or impractical to determine the ideographic advance measure,
it must be assumed to be 1em.
Tests
ic-unit-001.html
(live test)
(source)
ic-unit-002.html
(live test)
(source)
ic-unit-003.html
(live test)
(source)
ic-unit-004.html
(live test)
(source)
ic-unit-008.html
(live test)
(source)
ic-unit-009.html
(live test)
(source)
ic-unit-010.html
(live test)
(source)
ic-unit-011.html
(live test)
(source)
ic-unit-012.html
(live test)
(source)
ric
Equal to the value of the
ic
unit on the root element.
lh
Equal to the computed value of the
line-height
property of the element on which it is used,
converting
normal
to an absolute length
by using only the metrics of the
first available font
Tests
lh-rlh-on-root-001.html
(live test)
(source)
lh-unit-001.html
(live test)
(source)
lh-unit-002.html
(live test)
(source)
rlh
Equal to the value of the
lh
unit on the root element.
Note:
Setting the
height
of an element using either the
lh
or the
rlh
units
does not enable authors to control the actual number of lines in that element.
These units only enable length calculations based on the theoretical size of an ideal empty line;
the size of actual lines boxes may differ based on their content.
In cases where an author wants to limit the number of actual lines in an element,
the
max-lines
property can be used instead.
Tests
lh-rlh-on-root-001.html
(live test)
(source)
When used in the value of any
font-*
property on the element they refer to,
the
font-relative lengths
resolve against the computed metrics of the parent element—​or against the computed metrics corresponding to the initial values
of the
font
and
line-height
properties,
if the element has no parent.
Similarly,
when
lh
or
rlh
units are used
in the value of the
line-height
property or
font-*
properties on the element they refer to,
they resolve against the computed
line-height
and font metrics of the parent element—​or the computed metrics corresponding to the initial values
of the
font
and
line-height
properties,
if the element has no parent.
(The other font-relative lengths continue to resolve against
the element’s own metrics when used in
line-height
.)
When used outside the context of an element
(such as in
media queries
),
the
font-relative lengths
units refer to the metrics corresponding
to the initial values of the
font
and
line-height
properties.
Similarly, when specified in a document with no root element,
the
root font-relative lengths
are resolved
assuming the initial values of the
font
and
line-height
properties.
Note:
Font-relative units such as
ch
and
ic
can trigger font downloads,
if a required font is not yet loaded.
The
font-relative lengths
are calculated in the absence of shaping.
Some user-agents allow users to apply additional restrictions to font sizes in a document,
such as setting minimum font sizes to ensure readability.
Such restrictions must be applied to the
used value
of the affected properties only;
they
must not
affect the resolution of
font-relative lengths
used in properties.
However, in other contexts
(such as in
media queries
),
to the extent that they would impact the used font metrics,
such restrictions
do
affect the resolution of
font-relative lengths
Note:
In general, respecting a user’s preferences, like minimum font sizes,
is desirable;
it’s useful for a media query like
(min-width: 40em)
to use the actual font size the document will be displayed in.
However, having these preferences affect font-relative lengths
in properties on an element
was found to not be Web-compatible;
too many pages expect these units to be exact multiples of the specified
font-size
rather than the
actual
font-size after applying user preferences.
Some user-agents apply restrictions to the
line-height
values on form controls.
These must have no effect on the
lh
and
rlh
units.
The effect on their descendants, however, is
implementation-defined
6.1.2.
Viewport-percentage Lengths: the
*vw
*vh
*vi
*vb
*vmin
*vmax
units
The
viewport-percentage lengths
are relative to the size of the
initial containing block
—​which is itself based on the size of either
the viewport (for
continuous media
or the
page area
(for
paged media
).
When the height or width of the initial containing block is changed,
they are scaled accordingly.
6.1.2.1.
The Large, Small, and Dynamic Viewport Sizes
There are four variants of the
viewport-percentage length
units,
corresponding to three (possibly identical) notions of the viewport size.
large viewport
The
large viewport-percentage units
lv*
and
default viewport-percentage units
v*
are defined with respect to the
large viewport size
the viewport sized assuming
any
UA
interfaces that are dynamically expanded and retracted
to be retracted.
This allows authors to size content
such that it is guaranteed to fill the viewport,
noting that such content might be hidden behind such interfaces
when they are expanded.
The sizes of the
large viewport-percentage units
are fixed
(and therefore stable) unless the viewport itself is resized.
For example, on phones,
where screen real-estate is at a premium,
browsers will often hide part or all of the title and address bar
once the user starts scrolling the page.
The
large viewport-percentage units
are sized relative to
this larger everything-retracted space,
so content using these units will fill the entire visible page
when these UI elements are hidden.
However, when these retractable elements are shown,
they can obscure content
that is sized or positioned using these units.
small viewport
The
small viewport-percentage units
sv*
are defined with respect to the
small viewport size
the viewport sized assuming
any
UA
interfaces that are dynamically expanded and retracted
to be expanded.
This allows authors to size content
such that it can fit within the viewport
even when such interfaces are present,
noting that such content might not fill the viewport
when such interfaces are retracted.
The sizes of the
small viewport-percentage units
are fixed
(and therefore stable) unless the viewport itself is resized.
An element that is sized as
height: 100svh
, for example,
will fill the screen perfectly,
without any of its content being obscured,
when all the dynamic UI elements of the UA are shown.
Once those UI elements start being hidden, however,
there will be extra space around the element.
The
small viewport-percentage units
units are thus “safer” in general,
but might not produce the most attractive layout
once the user starts interacting with the page.
dynamic viewport
The
dynamic viewport-percentage units
dv*
are defined with respect to the
dynamic viewport size
the viewport sized
with dynamic consideration of any
UA
interfaces
that are dynamically expanded and retracted.
This allows authors to size content
such that it can exactly fit within the viewport
whether or not such interfaces are present.
The sizes of the
dynamic viewport-percentage units
are not stable
even while the viewport itself is unchanged.
Using these units can cause content
to resize e.g. while the user scrolls the page.
Depending on usage,
this can be disturbing to the user
and/or costly in terms of performance.
The UA is not required to animate the
dynamic viewport-percentage units
while expanding and retracting any relevant interfaces,
and may instead calculate the units
as if the relevant interface was fully expanded or retracted
during the UI animation.
(It is recommended that UAs assume the fully-retracted size
for this duration.)
Whether the expansion/retraction of a particular interface
(A)
changes the sizes of all of the
viewport-percentage lengths
(and the
initial containing block
simultaneously
or (B)
contributes to the differences between
the
large viewport size
and
small viewport size
is largely UA-dependent.
However:
Changes in interface that happen as a result of scrolling
or other frequent page interactions that would disturb the user
if they resulted in substantial layout changes
must be categorized as the latter (B).
Changes in interface that have a sufficiently steady state
that re-laying out the document into the adjusted space
would be beneficial to the user
must be categorized as the former (A).
Additionally,
UAs may have some dynamically-shown interfaces
that intentionally overlay content
and do not cause any shifts in layout—​and therefore have no effect on any of the
viewport-percentage lengths
(Typically on-screen keyboards will fit into this category.)
In all cases,
if the value of
overflow
or
scrollbar-gutter
on the
root element
in either axis
would cause scrollbars to appear (or space to be reserved for them) unconditionally
(for example,
overflow: scroll
, but not
overflow: auto
),
the
computed values
of the
viewport-percentage lengths
in that axis
are reduced in accordance with the
initial containing block
Otherwise,
and always in the case of
media queries
the
viewport-percentage lengths
are sized
assuming that scrollbars do not exist
(even if this diverges from the
initial containing block
).
Note:
The value of
overflow
on
the body element
can sometimes affect the presence of scrollbars on the
root element
This
does not
affect the size of viewport units, however.
6.1.2.2.
The Various Viewport-relative Units
The
viewport-percentage length
units are:
vw
svw
lvw
dvw
Equal to 1% of the width of the
large viewport size
small viewport size
large viewport size
and
dynamic viewport size
respectively.
In the example below, if the width of the viewport is 200mm,
the font size of
h1
elements will be 16mm
(i.e. (8×200mm)/100).
h1
font-size
vw
vh
svh
lvh
dvh
Equal to 1% of the height of the
large viewport size
small viewport size
large viewport size
and
dynamic viewport size
respectively.
vi
svi
lvi
dvi
Equal to 1% of the size of the
large viewport size
small viewport size
large viewport size
and
dynamic viewport size
(respectively)
in the box’s
inline axis
vb
svb
lvb
dvb
Equal to 1% of the size of the initial containing block
large viewport size
small viewport size
large viewport size
and
dynamic viewport size
(respectively)
in the box’s
block axis
vmin
svmin
lvmin
dvmin
Equal to the smaller of
*vw
or
*vh
vmax
svmax
lvmax
dvmax
Equal to the larger of
*vw
or
*vh
Tests
vh-calc-support-pct.html
(live test)
(source)
vh-calc-support.html
(live test)
(source)
vh-em-inherit.html
(live test)
(source)
vh-inherit.html
(live test)
(source)
vh-interpolate-pct.html
(live test)
(source)
vh-interpolate-px.html
(live test)
(source)
vh-interpolate-vh.html
(live test)
(source)
vh-support.html
(live test)
(source)
vh-support-margin.html
(live test)
(source)
vh-support-transform-origin.html
(live test)
(source)
vh-support-transform-translate.html
(live test)
(source)
vh-zero-support.html
(live test)
(source)
viewport-relative-lengths-scaled-viewport.html
(live test)
(source)
viewport-unit-011.html
(live test)
(source)
viewport-units-css2-001.html
(live test)
(source)
Note:
The original (unprefixed) viewport units
were defined
relative to the
initial containing block
which in
continuous media
always matched the (singular) viewport size.
The dynamism of browser chrome shifting in and out during scrolling was invented later,
and following Safari’s lead, most UAs mapped these units to the larger size.
Defining it this way is prettier in many cases,
but can also block critical content (such as toolbars, headers, and footers) in others.
It’s therefore not entirely clear whether this was the best mapping,
and thus earlier editions of this specifications
allowed UAs to choose the mapping of these default units.
However at this point the mapping to the
large viewport-percentage units
is presumed to be required for Web compatibility.
In situations where there is no element
or it hasn’t yet been styled
(such as when evaluating
media queries
),
the
*vi
and
*vb
units use the initial value of the
writing-mode
property
to determine which axis they correspond to.
6.2.
Absolute Lengths: the
cm
mm
in
pt
pc
px
units
The
absolute length units
are fixed in relation to each other
and
anchored
to some physical measurement.
They are mainly useful when the output environment is known.
The absolute units consist of
the
physical units
in
cm
mm
pt
pc
and the
visual angle unit (pixel unit)
px
):
unit
name
equivalence
cm
centimeters
1cm = 96px/2.54
mm
millimeters
1mm = 1/10th of 1cm
quarter-millimeters
1Q = 1/40th of 1cm
in
inches
1in = 2.54cm = 96px
pc
picas
1pc = 1/6th of 1in
pt
points
1pt = 1/72nd of 1in
px
pixels
1px = 1/96th of 1in
h1
margin
0.5
in
/* inches */
h2
line-height
cm
/* centimeters */
h3
word-spacing
mm
/* millimeters */
h3
letter-spacing
/* quarter-millimeters */
h4
font-size
12
pt
/* points */
h4
font-size
pc
/* picas */
font-size
12
px
/* px */
Note:
Lengths in publishing contexts
are sometimes written like
indicating a length of 2 picas and 3 points.
These can be written in CSS as
calc(2pc + 3pt)
(see
§ 10.1 Basic Arithmetic: calc()
).
All of the absolute length units are
compatible
and
px
is their
canonical unit
For a CSS device, these dimensions are
anchored
either
by relating the
physical units
to their physical measurements, or
by relating the
pixel unit
to the
reference pixel
For print media at typical viewing distances,
the
anchor unit
should be one of the
physical units
(inches, centimeters, etc).
For screen media (including high-resolution devices),
low-resolution devices,
and devices with unusual viewing distances,
it is recommended instead that the
anchor unit
be the
pixel unit
For such devices it is recommended that the
pixel unit
refer to the whole number of
device pixels
that best approximates the reference pixel.
Note:
If the
anchor unit
is the
pixel unit
the
physical units
might not match their physical measurements.
Alternatively if the
anchor unit
is a
physical unit
the
pixel unit
might not map to a whole number of
device pixels
Note:
This definition of the
pixel unit
and the
physical units
differs from the earlier editions of CSS1 and CSS2.
In particular, in previous versions of CSS the
pixel unit
and the
physical units
were not related by a fixed ratio:
the
physical units
were always tied to their physical measurements
while the
pixel unit
would vary to most closely match the reference pixel.
(This unfortunate change
was made because too much existing content relies on the assumption of 96dpi,
and breaking that assumption broke the content.)
Note:
Units are
ASCII case-insensitive
and serialize as lowercase, for example 1Q serializes as 1q.
Tests
absolute-length-units-001.html
(live test)
(source)
q-unit-case-insensitivity-001.html
(live test)
(source)
q-unit-case-insensitivity-002.html
(live test)
(source)
units-001.xht
(live test)
(source)
units-006.xht
(live test)
(source)
units-008.xht
(live test)
(source)
units-009.xht
(live test)
(source)
The
reference pixel
is the visual angle of one pixel on a device with a
device pixel
density of 96dpi
and a distance from the reader of an arm’s length.
For a nominal arm’s length of 28 inches,
the visual angle is therefore about 0.0213 degrees.
For reading at arm’s length,
1px thus corresponds to about 0.26 mm (1/96 inch).
The image below illustrates the effect of viewing distance on the size of a reference pixel:
a reading distance of 71 cm (28 inches)
results in a reference pixel of 0.26 mm,
while a reading distance of 3.5 m (12 feet)
results in a reference pixel of 1.3 mm.
Showing that pixels must become larger if the viewing distance increases
This second image illustrates the effect of a device’s resolution on the pixel unit:
an area of 1px by 1px is covered by a single dot in a low-resolution device
(e.g. a typical computer display),
while the same area is covered by 16 dots in a higher resolution device
(such as a printer).
Showing that more device pixels (dots) are needed
to cover a 1px by 1px area on a high-resolution device
than on a lower-resolution one
(of the same approximate viewing distance)
Tests
absolute-length-units-001.html
(live test)
(source)
device pixel
is the smallest unit of area on the device output
capable of displaying its full range of colors.
For typical color screens, it’s a square or somewhat rectangular region
containing a red, green, and blue subpixel.
Many non-traditional outputs exist that can blur this definition,
such as by displaying some colors at higher resolutions.
Such devices still expose some equivalent notion of "device pixel",
however.
7.
Other Quantities
7.1.
Angle Units: the

type and
deg
grad
rad
turn
units
Angle values are

s denoted by

The angle unit identifiers are:
deg
Degrees. There are 360 degrees in a full circle.
grad
Gradians, also known as "gons" or "grades".
There are 400 gradians in a full circle.
rad
Radians. There are 2π radians in a full circle.
turn
Turns. There is 1 turn in a full circle.
For example, a right angle is
90deg
or
100grad
or
0.25turn
or
approximately
1.57rad
All

units are
compatible
and
deg
is their
canonical unit
By convention,
when an angle denotes a direction in CSS,
it is typically interpreted as a
bearing angle
where 0deg is "up" or "north" on the screen,
and larger angles are more clockwise
(so 90deg is "right" or "east").
For example, in the
linear-gradient()
function,
the

that determines the direction of the gradient
is interpreted as a bearing angle.
Note:
For legacy reasons,
some uses of

allow a bare
to mean
0deg
This is not true in general, however,
and will not occur in future uses of the

type.
Tests
angle-units-001.html
(live test)
(source)
angle-units-002.html
(live test)
(source)
angle-units-003.html
(live test)
(source)
angle-units-004.html
(live test)
(source)
angle-units-005.html
(live test)
(source)
calc-angle-values.html
(live test)
(source)
7.2.
Duration Units: the

type and
ms
units
Time values are
dimensions
denoted by

The time unit identifiers are:
Seconds.
ms
Milliseconds. There are 1000 milliseconds in a second.
All

units are
compatible
and
is their
canonical unit
Properties may restrict the time value to some range.
If the value is outside the allowed range,
the declaration is invalid and must be
ignored
Tests
calc-time-values.html
(live test)
(source)
transition-delay-001.html
(live test)
(source)
transition-duration-001.html
(live test)
(source)
7.3.
Frequency Units: the

type and
Hz
kHz
units
Frequency values are
dimensions
denoted by

The frequency unit identifiers are:
Hz
Hertz. It represents the number of occurrences per second.
kHz
KiloHertz. A kiloHertz is 1000 Hertz.
For example, when representing sound pitches, 200Hz (or 200hz)
is a bass sound, and 6kHz (or 6khz) is a treble sound.
All

units are
compatible
and
hz
is their
canonical unit
Note:
Units are
ASCII case-insensitive
and serialize as lowercase, for example 1Hz serializes as 1hz.
7.4.
Resolution Units: the

type and
dpi
dpcm
dppx
units
Resolution units are
dimensions
denoted by

The resolution unit identifiers are:
dpi
Dots per inch.
dpcm
Dots per centimeter.
dppx
Dots per
px
unit.
The

unit represents the size of a single "dot" in a graphical representation
by indicating how many of these dots fit in a CSS
in
cm
, or
px
For uses, see e.g. the
resolution
media query in
[MEDIAQ]
or the
image-resolution
property defined in
[CSS3-IMAGES]
All

units are
compatible
and
dppx
is their
canonical unit
The allowed range of

values
always
excludes negative values,
in addition to any explicit ranges that might be specified.
Note that due to the 1:96 fixed ratio of CSS
in
to CSS
px
1dppx
is equivalent to
96dpi
This corresponds to the default resolution of images displayed in CSS:
see
image-resolution
The following @media rule uses Media Queries
[MEDIAQ]
to assign some special style rules to devices that use two or more device pixels per CSS
px
unit:
@media
min-resolution:
dppx
...
8.
Data Types Defined Elsewhere
Some data types are defined in their own modules.
This example talks about some of the most common ones
used across several specifications.
8.1.
Colors: the

type
The

data type is defined in
[CSS-COLOR-4]
UAs must interpret

as defined therein.
8.1.1.
Combination of

Interpolation
of

is defined in
CSS Color 4
§  13. Color Interpolation
Interpolation is done between premultiplied colors,
as defined in
CSS Color 4
§ 13.3 Interpolating with Alpha
The

type is
not additive
Note:
the CSS WG is interested to
hear
use-cases for addition of

and may consider making

additive in the future.
8.2.
Images: the

type
The

data type is defined in
[CSS3-IMAGES]
UAs that support CSS Images Level 3 or its successor
must interpret

as defined therein.
UAs that do not yet support CSS Images Level 3
must interpret

as

8.2.1.
Combination of

Note:
Interpolation of

is defined in
CSS Images 3
§ 6 Interpolation
Images are
not additive
8.3.
2D Positioning: the

type
The

value specifies the position of a object area (e.g. background image)
inside a positioning area (e.g. background positioning area).
It is computed and interpreted as specified for
background-position
[CSS3-BACKGROUND]

left
center
right
top
bottom

left
center
right
&&
top
center
bottom
left
center
right

top
center
bottom

left
right

&&
top
bottom

Note:
The
background-position
property also accepts a three-value syntax.
This has been disallowed generically because it creates parsing ambiguities
when combined with other length or percentage components in a property value.
8.3.1.
Parsing

When specified in a grammar alongside other keywords,

s, or

s,

is
greedily
parsed;
it consumes as many components as possible.
For example,
transform-origin
defines a 3D position
as (effectively)


A value such as
left 50px
will be parsed as a 2-value

with an omitted z-component;
on the other hand,
a value such as
top 50px
will be parsed as a single-value

followed by a

8.3.2.
Serializing

When serializing the
specified value
of a

If only one component is specified:
The implied
center
keyword is added,
and a 2-component value is serialized.
If two components are specified:
Keywords are serialized as keywords.

s are serialized as

s.
Components are serialized horizontal first, then vertical.
If four components are specified:
Keywords and offsets are both serialized.
Components are serialized horizontal first, then vertical.
Note:

values are never serialized as a single value,
even when a single value would produce the same behavior,
to avoid causing parsing ambiguities in some grammars
where a

is placed next to a

such as
transform-origin
Note:
Computed values
are always serialized as two offsets (without keywords)
because the
computed value
does not preserve syntactic distinctions.
8.3.3.
Combination of

Interpolation
of

is defined as
the independent interpolation of each component (x, y)
normalized as an offset from the top left corner
as a

Addition
of

is likewise defined as
the independent
addition
each component (x, y)
normalized as an offset from the top left corner
as a

9.
Functional Notations
functional notation
is a type of component value
that can represent more complex types or invoke special processing.
The syntax starts with the name of the function
immediately followed by a left parenthesis
(i.e. a

followed by the argument(s) to the notation
followed by a right parenthesis.
Like keywords, function names are
ASCII case-insensitive
White space
is allowed, but optional,
immediately inside the parentheses.
Functions can take multiple arguments,
which are formatted similarly to a CSS property value.
See
§ 2.6 Functional Notation Definitions
Note:
Some legacy
functional notations
, such as
rgba()
, use commas unnecessarily,
but generally commas are only used to separate items in a list,
or pieces of a grammar that would be ambiguous otherwise.
If a comma is used to separate arguments,
white space
is optional before and after the comma.
background
url
);
color
rgb
100
200
50
);
content
counter
list-item
". "
width
calc
50
em
);
The
math functions
are defined below.
Other
functional notations
are defined in their own modules;
for example the

functions are defined in
[CSS-COLOR-4]
and
[CSS-COLOR-5]
10.
Mathematical Expressions
The
math functions
calc()
clamp()
sin()
and others defined in this chapter)
allow numeric CSS values
to be written as mathematical expressions.
math function
represents a numeric value,
one of:









...or the

/etc mixed types,
and can be used wherever such a value would be valid.
10.1.
Basic Arithmetic:
calc()
The
calc()
function is a
math function
that allows basic arithmetic to be performed on numerical values,
using addition (
), subtraction (
), multiplication (
), division (
),
and parentheses.
calc()
function contains a single
calculation
which is a sequence of values interspersed with operators,
and possibly grouped by parentheses
(matching the

grammar),
which represents the result of evaluating the expression
using standard operator precedence rules
and
bind tighter than
and
and operators are otherwise evaluated left-to-right).
The
calc()
function represents the result of its contained
calculation
Components of a
calculation
can be literal values
(such as
5px
),
other
math functions
or other expressions, such as
var()
that evaluate to a valid argument type (like

).
Math functions
can be used to combine value that use different units.
In this example the author wants the
margin box
of each section
to take up 1/3 of the space,
so they start with
100%/3
then subtract the element’s borders and margins.
box-sizing
can automatically achieve this effect for borders and padding,
but a
math function
is needed if you want to include margins.)
section
float
left
margin
em
border
solid
px
width
calc
100
em
px
);
Similarly, in this example the gradient will show a color transition
only in the first and last
20px
of the element:
.fade
background-image
linear-gradient
silver
white
20
px
white
calc
100
20
px
),
silver
100
);
Math functions
can also be useful just to express values
in a more natural, readable fashion,
rather than as an obscure decimal.
For example, the following sets the
font-size
so that exactly 35em fits within the viewport,
ensuring that roughly the same amount of text always fills the screen no matter the screen size.
:root
font-size
calc
100
vw
35
);
Functionality-wise, this is identical to just writing
font-size: 2.857vw
but then the intent
(that
35em
fills the viewport)
is much less clear to someone reading the code;
the later reader will have to reverse the math themselves
to figure out that 2.857 is meant to approximate 100/35.
Standard mathematical precedence rules for the operators apply:
calc(2 + 3 * 4)
is equal to
14
not
20
Parentheses can be used to manipulate precedence:
calc((2 + 3) * 4)
is instead equal to
20
Parentheses and nesting additional
calc()
functions are equivalent;
the preceding expression could equivalently have been written as
calc(calc(2 + 3) * 4)
This can be useful when building up values piecemeal via
var()
such as in the following example:
.aspect-ratio-box
--ar
calc
16
);
--w
calc
100
);
--h
calc
var
--w
var
--ar
));
width
var
--w
);
height
var
--h
);
Although
--ar
could
have been written as simply
--ar: (16 / 9);
--w
is used both on its own (in
width
and as a
calc()
component (in
--h
),
so it has to be written as a full
calc()
function itself.
Tests
calc-ch-ex-lang.html
(live test)
(source)
calc-in-color-001.html
(live test)
(source)
calc-in-font-feature-settings.html
(live test)
(source)
calc-rem-lang.html
(live test)
(source)
calc-rounding-001.html
(live test)
(source)
ex-calc-expression-001.html
(live test)
(source)
10.2.
Comparison Functions:
min()
max()
, and
clamp()
The comparison functions of
min()
max()
, and
clamp()
compare multiple
calculations
and represent the value of one of them.
The
min()
or
max()
functions
contain one or more comma-separated
calculations
and represent the smallest (most negative) or largest (most positive) of them, respectively.
The
clamp()
function takes three
calculations
—​a minimum value, a central value, and a maximum value—​and represents its central calculation,
clamped according to its min and max calculations,
favoring the min calculation if it conflicts with the max.
(That is, given
clamp(MIN, VAL, MAX)
it represents exactly the same value as
max(MIN, min(VAL, MAX))
).
Either the min or max calculations
(or even both)
can instead be the keyword
none
which indicates the value is
not
clamped from that side.
(That is,
clamp(MIN, VAL, none)
is equivalent to
max(MIN, VAL)
clamp(none, VAL, MAX)
is equivalent to
min(VAL, MAX)
and
clamp(none, VAL, none)
is equivalent to just
calc(VAL)
.)
For all three functions,
the argument
calculations
can resolve to any


, or

but must have a
consistent type
or else the function is invalid;
the result’s type will be the
consistent type
min()
max()
, and
clamp()
can be used to make sure a value doesn’t exceed a "safe" limit:
For example, "responsive type" that sets
font-size
with viewport units
might still want a minimum size to ensure readability:
.type
/* Set font-size to 10x the average of vw and vh,
but don't let it go below 12px. */
font-size:
max
10
vw
vh
12
px
);
Note:
Full math expressions are allowed in each of the arguments;
there’s no need to nest a
calc()
inside!
You can also provide more than two arguments,
if you have multiple constraints to apply.
An occasional point of confusion when using
min()
max()
is that you use
max()
to impose a minimum value on something
(that is, properties like
min-width
effectively use
max()
),
and
min()
to impose a maximum value on something;
it’s easy to accidentally reach for the opposite function
and try to use
min()
to add a minimum size.
Using
clamp()
can make the code read more naturally,
as the value is nestled between its minimum and maximum:
.type
/* Force the font-size to stay between 12px and 100px */
font-size:
clamp
12
px
10
vw
vh
100
px
);
Or, if you only wanted to impose a minimum size,
but allow the font-size to grow as large as it wants:
.type
/* Force the font-size to be at least 12px */
font-size:
clamp
12
px
10
vw
vh
none
);
Note that
clamp()
matching CSS conventions elsewhere,
has its minimum value "win" over its maximum value
if the two are in the "wrong order".
That is,
clamp(100px, ..., 50px)
will resolve to
100px
exceeding its stated "max" value.
If alternate resolution mechanics are desired
they can be achieved by combining
clamp()
with
min()
or
max()
To have MAX win over MIN:
clamp(min(MIN, MAX), VAL, MAX)
If you want to avoid repeating the MAX calculation,
you can just reverse the nesting of functions that
clamp()
is defined against—​
min(MAX, max(MIN, VAL))
To have MAX and MIN "swap" when they’re in the wrong order:
clamp(min(MIN, MAX), VAL, max(MIN, MAX))
Unfortunately, there’s no easy way to do this without repeating the MIN and MAX terms.
10.3.
Stepped Value Functions:
round()
mod()
, and
rem()
The stepped-value functions,
round()
mod()
, and
rem()
all transform a given value
according to another "step value",
in different ways.
The
round(

?, A, B?)
function
contains an optional rounding strategy,
and two
calculations
A and B,
and returns the value of A,
rounded according to the rounding strategy,
to the nearest integer multiple of B either above or below A.
The argument
calculations
can resolve to any


, or

but must have a
consistent type
or else the function is invalid;
the result’s type will be the
consistent type
If A is exactly equal to an integer multiple of B,
round()
resolves to A exactly
(preserving whether A is 0⁻ or 0⁺, if relevant).
Otherwise, there are two integer multiples of B that are potentially "closest" to A,
lower B
which is closer to −∞
and
upper B
which is closer to +∞.
The following

s dictate how to choose between them:
nearest
Choose whichever of
lower B
and
upper B
that has the smallest absolute difference from A.
If both have an equal difference
(A is exactly between the two values),
choose
upper B
up
Choose
upper B
down
Choose
lower B
to-zero
Choose whichever of
lower B
and
upper B
that has the smallest absolute difference from 0.
line-width
If A is non-negative and B is omitted,
A is
snapped as a border width
Otherwise, identical to
nearest
except that if A is positive (non-zero)
and one of
lower B
or
upper B
is zero,
choose the non-zero one,
then
snap as a border width
If
lower B
would be zero,
it is specifically equal to 0⁺;
if
upper B
would be zero,
it is specifically equal to 0⁻.
If

is omitted,
it defaults to
nearest
(Aka
rounding to the nearest integer
.)
If

is
line-width
the
type
of A
must match

If the
type
of A
matches

then B may be omitted,
and defaults to
If

is
line-width
B may also be omitted
and has special behavior (defined above)
essentially defaulting it to one device pixel.
In all other cases,
omitting B is invalid.
While
round(line-width, ...)
attempts to ensure that non-zero values never round to zero,
passing a sufficiently small A value
might cause it to be represented as 0 internally anyway.
Use caution when writing very small values;
round(line-width, 0.1px)
is safer than
round(line-width, 0.0001px)
and the difference between the two is almost certainly nil in practice.
CSSOM needs to specify how it rounds,
and it’s probably good for CSS functions to round the same way by default.
What behavior should be used?
[Issue #5689]
Unlike languages like JavaScript
which have a natural "precision" to round to
(integers),
CSS values have no such precision
because values can be written in many different compatible units.
As such, the precision has to be given explicitly;
to round a width to the nearest
50px
one can write
round(var(--width), 50px)
Note:
JavaScript and other programming languages
sometimes separate out the rounding strategies into separate rounding functions.
JS’s
Math
floor
()
is equivalent to CSS’s
round(down, ...)
JS’s
Math
ceil
()
is equivalent to CSS’s
round(up, ...)
JS’s
Math
trunc
()
is equivalent to CSS’s
round(to-zero, ...)
and JS’s
Math
round
()
is equivalent to CSS’s
round(nearest, ...)
or just
round(...)
Note:
The

keywords
are the same as the keywords in
block-step-size
and have the same behavior.
block-step-size
just lacks
to-zero
since block sizes are always non-negative,
to-zero
and
down
would be identical.)
The modulus functions
mod(A, B)
and
rem(A, B)
similarly contain two
calculations
A and B,
and return the difference between A
and the nearest integer multiple of B either above or below A.
The argument
calculations
can resolve to any


, or

but must have the
same
type
or else the function is invalid;
the result will have the same
type
as the arguments.
The two functions are very similar,
and in fact return identical results
if both arguments are positive or both are negative:
the value of the function is equal to the value of A
shifted by the integer multiple of B
that brings the value
between zero and B
(Specifically, the range includes zero
and excludes B.
More specifically, if B is positive the range starts at 0⁺,
and if B is negative it starts at 0⁻.)
For example,
mod(18px, 5px)
resolves to the value
3px
because subtracting
5px * 3
from
18px
yields
3px
which is the only such value between
0px
and
5px
Similarly,
mod(-140deg, -90deg)
resolves to the value
-50deg
because adding
-90deg * 1
to
-140deg
yields
-50deg
which is the only such value between
0deg
and
-90deg
Evaluating either of these examples with
rem()
yields the exact same results.
Their behavior diverges if the A value and the B step
are on opposite sides of zero:
mod()
(short for “modulus”)
continues to choose the integer multiple of B
that puts the value
between zero and B
, as above
(guaranteeing that the result will either be zero
or share the sign of B, not A),
while
rem()
(short for "remainder")
chooses the integer multiple of B
that puts the value
between zero and -B
avoiding changing the sign of the value.
For example,
mod(-18px, 5px)
resolves to the value
2px
adding
5px * 4
to
-18px
yields
2px
which is between
0px
and
5px
On the other hand,
rem(-18px, 5px)
resolves to the value
-3px
adding
5px * 3
to
-18px
yields
-3px
which has the same sign as
-18px
but is between
0px
and
-5px
Similarly,
mod(140deg, -90deg)
resolves to the value
-40deg
(adding
-90deg * 2
to
140deg
bringing it to between
0deg
and
-90deg
),
but
rem(140deg, -90deg)
resolves to the value
50deg
When should I choose
mod()
vs
rem()
Typically, users of this operation
are in control of the step value (B),
and are modifying an unknown value A.
As a result, it’s
usually
more expected
that the result is between 0 and B,
regardless of A’s sign,
meaning
mod()
should be chosen.
For example,
if an author wants to know whether a length
is an even or odd number of pixels,
mod(A, 2px)
will return either
0px
or
1px
(assuming the value is a whole number of pixels to begin with),
regardless of the value of a.
rem(A, 2px)
, on the other hand,
will return
0px
if A is an even number of pixels,
but will return
either
1px
or
-1px
if it’s odd,
depending on whether A is positive or negative.
The opposite situation does sometimes occur,
however,
and so
rem()
is provided to cater to that.
As well,
rem()
is the behavior of JavaScript’s
operator,
so if an exact match between CSS and JS code is desired,
rem()
can be useful.
Note:
mod()
and
rem()
can also be defined directly in terms of other functions:
mod(A, B)
is equivalent to
calc(A - sign(B)*round(down, A*sign(B), B))
(a hacky way to say "round(down) when B is positive, round(up) when B is negative),
while
rem(A, B)
is equivalent to
calc(A - round(to-zero, A, B))
(These expressions don’t always handle 0⁺ and 0⁻ correctly, though,
because 0⁻ semantics aren’t commutative for addition.)
10.3.1.
Argument Ranges
In
round(A, B)
if B is 0,
the result is NaN.
If A and B are both infinite,
the result is NaN.
If A is infinite but B is finite,
the result is the same infinity.
If A is finite but B is infinite,
the result depends on the

and the sign of A:
nearest
to-zero
If A is positive or 0⁺, return 0⁺.
Otherwise, return 0⁻.
up
If A is positive (not zero), return +∞.
If A is 0⁺, return 0⁺.
Otherwise, return 0⁻.
down
If A is negative (not zero), return −∞.
If A is 0⁻, return 0⁻.
Otherwise, return 0⁺.
In
mod(A, B)
or
rem(A, B)
if B is 0,
the result is NaN.
If A is infinite,
the result is NaN.
In
mod(A, B)
only,
if B is infinite
and A has opposite sign to B
(including an oppositely-signed zero),
the result is NaN.
Note:
All other "infinite B" cases are valid,
and just return A immediately.
10.4.
Trigonometric Functions:
sin()
cos()
tan()
asin()
acos()
atan()
, and
atan2()
The trigonometric functions—​
sin()
cos()
tan()
asin()
acos()
atan()
, and
atan2()
—​compute the various basic trigonometric relationships.
The
sin(A)
cos(A)
, and
tan(A)
functions
all contain a single
calculation
which must resolve to either a

or an

and compute their corresponding function
by interpreting the result of their argument as radians.
(That is,
sin(45deg)
sin(.125turn)
, and
sin(3.14159 / 4)
all represent the same value,
approximately
.707
.)
They all represent a

with the return type
made consistent
with the input
calculation’s
type.
sin()
and
cos()
will always return a number between −1 and 1,
while
tan()
can return any number between −∞ and +∞.
(See
§ 10.9 Type Checking
for details on how
math functions
handle ∞.)
The
asin(A)
acos(A)
, and
atan(A)
functions
are the "arc" or "inverse" trigonometric functions,
representing the inverse function to their corresponding "normal" trig functions.
All of them contain a single
calculation
which must resolve to a

and compute their corresponding function,
interpreting their result as a number of radians,
representing an

with the return type
made consistent
with the input
calculation’s
type.
The angle returned by
asin()
must be normalized to the range [
-90deg
90deg
];
the angle returned by
acos()
to the range [
0deg
180deg
];
and the angle returned by
atan()
to the range [
-90deg
90deg
].
The
atan2(A, B)
function
contains two comma-separated
calculations
, A and B.
A and B can resolve to any


, or

but must have a
consistent type
or else the function is invalid.
The function returns the

between the positive X-axis and the point (B,A),
with the return type
made consistent
with the input
calculation’s
type.
The returned angle must be normalized to the interval (
-180deg
180deg
(that is, greater than
-180deg
, and less than or equal to
180deg
).
Note:
atan2(Y, X)
is
generally
equivalent to
atan(Y / X)
but it gives a better answer when the point in question may include negative components.
atan2(1, -1)
, corresponding to the point (-1, 1),
returns
135deg
distinct from
atan2(-1, 1)
, corresponding to the point (1, -1),
which returns
-45deg
In contrast,
atan(1 / -1)
and
atan(-1 / 1)
both return
-45deg
because the internal calculation resolves to
-1
for both.
10.4.1.
Argument Ranges
In
sin(A)
cos(A)
, or
tan(A)
if A is infinite,
the result is NaN.
(See
§ 10.9 Type Checking
for details on how
math functions
handle NaN.)
In
sin(A)
or
tan(A)
if A is 0⁻,
the result is 0⁻.
In
tan(A)
, if A is one of the asymptote values
(such as
90deg
270deg
, etc),
the numeric result is
implementation-defined
If an implementation is capable of exactly representing these inputs,
it
should
return
+∞ for the asymptotes at
90
deg
+ N*
360
deg
and −∞ for the asymptotes at
-90
deg
+ N*
360
deg
but implementations are not required to be able to exactly represent these inputs
(and if they can’t, will return whatever the correct numeric answer is
for the closest approximation to the input
they are capable of representing).
Authors
must not
rely on
tan()
returning any particular value
for these inputs.
Why are these
implementation-defined
The tangent function is
discontinuous
at its asymptotes:
it approaches infinity from one side
and
negative infinity from the other side,
and isn’t defined at the exact values of the asymptote.
Further, whether or not the asymptotic values
are exactly representable in implementations
depends on how they internally store and manipulate angles;
when written in degrees the values are simple (
90deg
, etc),
but in radians the values are transcendental (
pi / 2
, etc)
and cannot be exactly represented.
So, even defining a specific behavior for these values is difficult;
if an implementation uses radians internally,
it would have to do some fuzzy matching
to return the defined value
when the input is
sufficiently close
to the asymptote.
The other major language for the Web, JavaScript,
exposes these functions as taking radians only,
so it can’t hit the exact asymptotes either
(and this true for most other computer languages, too).
Authors writing code in JS, then,
can’t rely on any specific behavior for these values either,
and it’s unlikely that their needs in CSS are significantly different.
The suggested behavior for implementations that can exactly represent the asymptote values
preserves round-tripping with the
atan()
function:
tan(atan(X))
and
atan(tan(X))
will both return (approximately) X
for all possible X values,
given this definition.
It also means that within the supported output range of
atan()
the function is continuous.
In
asin(A)
or
acos(A)
if A is less than -1 or greater than 1,
the result is NaN.
In
acos(A)
if A is exactly 1,
the result is 0.
In
asin(A)
or
atan(A)
if A is 0⁻,
the result is 0⁻.
In
atan(A)
if A is +∞,
the result is
90deg
if A is −∞,
the result is
-90deg
In
atan2(Y, X)
the following table gives the results for all unusual argument combinations:
−∞
-finite
0⁻
0⁺
+finite
+∞
−∞
-135deg
-90deg
-90deg
-90deg
-90deg
-45deg
-finite
-180deg
(normal)
-90deg
-90deg
(normal)
0⁻deg
0⁻
-180deg
-180deg
-180deg
0⁻deg
0⁻deg
0⁻deg
0⁺
180deg
180deg
180deg
0⁺deg
0⁺deg
0⁺deg
+finite
180deg
(normal)
90deg
90deg
(normal)
0⁺deg
+∞
135deg
90deg
90deg
90deg
90deg
45deg
Note:
All of these behaviors are intended to match the "standard" definitions of these functions
as implemented by most programming languages,
in particular as implemented in JS.
10.5.
Exponential Functions:
pow()
sqrt()
hypot()
log()
exp()
The exponential functions—​
pow()
sqrt()
hypot()
log()
, and
exp()
—​compute various exponential functions with their arguments.
The
pow(A, B)
function
contains two comma-separated
calculations
A and B,
both of which must resolve to

s,
and returns the result of raising A to the power of B,
returning the value as a

The input
calculations
must have a
consistent type
or else the function is invalid;
the result’s type will be the
consistent type
The
sqrt(A)
function
contains a single
calculation
which must resolve to a

and returns the square root of the value
as a

with the return type
made consistent
with the input
calculation’s
type.
sqrt(X)
and
pow(X, .5)
are basically equivalent,
differing only in some error-handling;
sqrt()
is a common enough function
that it is provided as a convenience.)
The
hypot(A, …)
function
contains one or more comma-separated
calculations
and returns the length of an N-dimensional vector
with components equal to each of the
calculations
(That is,
the square root of the sum of the squares of its arguments.)
The argument
calculations
can resolve to any


, or

but must have a
consistent type
or else the function is invalid;
the result’s type will be the
consistent type
Why does
hypot()
allow dimensions (values with units), but
pow()
and
sqrt()
only work on numbers?
You are allowed to write expressions like
hypot(30px, 40px)
which resolves to
50px
but you aren’t allowed to write the expression
sqrt(pow(30px, 2) + pow(40px, 2))
despite the two being equivalent in most mathematical systems.
There are two reasons for this:
numeric precision in the exponents,
and clashing expectations from authors.
First, numerical precision.
For a
type
to
match
a CSS production like

it needs to have a single unit with its exponent set to exactly 1.
Theoretically, expressions like
pow(pow(30px, 3), 1/3)
should result in exactly that:
the inner
pow(30px, 3)
would resolve to a value of 27000 with a
type
of «[ "length" → 3 ]»
(aka

³),
and then the
pow(X, 1/3)
would cube-root the value back down to 30 and multiply the exponent by 1/3,
giving «[ "length" → 1 ]»,
which
matches

In the realm of pure mathematics, that’s guaranteed to work out;
in the real-world of computers using binary floating-point arithmetic,
in some cases the powers might not exactly cancel out,
leaving you with an invalid
math function
for confusing, hard-to-track-down reasons.
(For a JS example,
evaluate
Math.
pow
Math.
pow
30
10
),
.1+.1+.1
the result is not exactly 30,
because
.1+.1+.1
is not exactly 3/10.
Instead,
10
.1
.1
.1
is
slightly greater
than 1.)
Requiring authors to cast their value down into a number,
do all the math on the raw number,
then finally send it back to the desired unit,
while inconvenient,
ensures that numerical precision won’t bite anyone:
calc(pow(pow(30px / 1px, 3), 1/3) * 1px)
is guaranteed to resolve to a

with a value that, if not exactly 30, is at least very close to 30,
even if numerical precision actually prevents the powers from exactly canceling.
Second, clashing expectations.
It’s not uncommon for authors to expect
pow(30px, 2)
to result in
900px
(such as in
this Sass issue
);
that is,
just squaring the numerical value
and leaving the unit alone.
This, however, means the result is dependent on what unit you’re expressing the argument in;
if
1em
is
16px
then
pow(1em, 2)
would give
1em
while
pow(16px, 2)
would give
256px
, or
16em
which are very different values for what should otherwise be identical input arguments!
This sort of input dependency is troublesome for CSS,
which generally allows values to be
canonicalized
freely;
it also makes more complex expressions like
pow(2em + 10px, 2)
difficult to interpret.
Again, requiring authors to cast their value down into a number
and then back up again into the desired unit
sidesteps these issues;
pow(30, 2)
is indeed
900
and the author can interpret that however they wish.
On the other hand,
hypot()
doesn’t suffer from these problems.
Numerical precision in units isn’t a concern,
as the inputs and output all have the same type.
The result isn’t unit-dependent, either,
due to the nature of the operation;
hypot(3em, 4em)
and
hypot(48px, 64px)
both result in the same length
when
1em
equals
16px
5em
or
80px
Thus it’s fine to let author use dimensions directly in
hypot()
The
log(A, B?)
function
contains one or two
calculations
(representing the value to be logarithmed,
and the base of the logarithm,
defaulting to e),
which must resolve to

s,
and returns the logarithm base B of the value A,
as a

with the return type
made consistent
with the input
calculation’s
type.
The
exp(A)
function
contains one
calculation
which must resolve to a

and returns the same value as
pow(e, A)
as a

with the return type
made consistent
with the input
calculation’s
type.
The
pow()
function can be useful for strategies like
CSS Modular Scale
which relates all the font-sizes on a page to each other by a fixed ratio.
These sizes can be easily written into custom properties like:
:root
--h6
calc
rem
pow
1.5
-1
));
--h5
calc
rem
pow
1.5
));
--h4
calc
rem
pow
1.5
));
--h3
calc
rem
pow
1.5
));
--h2
calc
rem
pow
1.5
));
--h1
calc
rem
pow
1.5
));
...rather than writing out the values in pre-calculated numbers like
5.0625rem
(what
calc(1rem * pow(1.5, 4))
resolves to)
which have less clear provenance when encountered in a stylesheet.
With a single argument,
hypot()
gives the absolute value of its input;
hypot(2em)
and
hypot(-2em)
both resolve to
2em
With more arguments,
it gives the size of the main diagonal of a box
whose side lengths are given by the arguments.
This can be useful for transform-related things,
giving the distance that an element will actually travel
when it’s translated by a particular X, Y, and Z amount.
For example,
hypot(30px, 40px)
resolves to
50px
which is indeed the distance between an element’s starting and ending positions
when it’s translated by a
translate(30px, 40px)
transform.
If an author wanted elements to get smaller as they moved further away from their starting point
(drawing some sort of word cloud, for example),
they could then use this distance in their scaling factor calculations.
With a single argument,
log()
provides the “natural log” of its argument,
or the log base e,
same as JavaScript.
If one instead wants log base 10
(to, for example, count the number of digits in a value)
or log base 2
(counting the number of bits in a value),
log(X, 10)
or
log(X, 2)
provide those values.
10.5.1.
Argument Ranges
In
pow(A, B)
if A is negative and finite,
and B is finite,
B must be an integer,
or else the result is NaN.
If A or B are infinite or 0,
the following tables give the results:
A is −∞
A is 0⁻
A is 0⁺
A is +∞
B is −finite
0⁻ if B is an odd integer, 0⁺ otherwise
−∞ if B is an odd integer, +∞ otherwise
+∞
0⁺
B is 0
always 1
B is +finite
−∞ if B is an odd integer, +∞ otherwise
0⁻ if B is an odd integer, 0⁺ otherwise
0⁺
+∞
A is < -1
A is -1
-1 < A < 1
A is 1
A is > 1
B is +∞
result is +∞
result is NaN
result is 0⁺
result is NaN
result is +∞
B is −∞
result is 0⁺
result is NaN
result is +∞
result is NaN
result is 0⁺
In
sqrt(A)
if A is +∞,
the result is +∞.
If A is 0⁻,
the result is 0⁻.
If A is less than 0,
the result is NaN.
In
hypot(A, …)
if any of the inputs are infinite,
the result is +∞.
In
log(A, B)
if B is 1 or negative,
B values
between
0 and 1,
or greater than 1,
are valid.
the result is NaN.
If A is negative,
the result is NaN.
If A is 0⁺ or 0⁻,
the result is −∞.
If A is 1,
the result is 0⁺.
If A is +∞,
the result is +∞.
In
exp(A)
if A is +∞,
the result is +∞.
If A is −∞,
the result is 0⁺.
(See
§ 10.9 Type Checking
for details on how
math functions
handle NaN and infinities.)
All of these behaviors are intended to match the "standard" definitions of these functions
as implemented by most programming languages,
in particular as implemented in JS.
The only divergences from the behavior of the equivalent JS functions
are that NaN is "infectious" in
every
function,
forcing the function to return NaN if any argument calculation is NaN.
Details of the JS Behavior
There are two cases in JS where a NaN is not "infectious"
to the math function it finds itself in:
Math
hypot
Infinity
NaN
will return
Infinity
Math
pow
NaN
will return
The logic appears to be that,
if you replace the NaN with
any
Number,
the return value will be the same.
However, this logic is not applied consistently to the
Math
functions:
Math
max
Infinity
NaN
returns
NaN
, not
Infinity
the same is true of
Math
min
Infinity
NaN
Because this is an error corner case,
JS isn’t consistent on the matter,
and NaN recognition/handling of
calculations
is likely done at a higher CSS level
rather than in the internal math functions anyway,
consistency in CSS was chosen to be more important,
so all functions were defined to have "infectious" NaN.
10.6.
Sign-Related Functions:
abs()
sign()
The sign-related functions—​
abs()
and
sign()
—​compute various functions related to the sign of their argument.
The
abs(A)
function
contains one
calculation
A,
and returns the absolute value of A,
as the same
type
as the input:
if A’s numeric value is positive or 0⁺, just A again;
otherwise
-1 * A
The
sign(A)
function
contains one
calculation
A,
and returns -1 if A’s numeric value is negative,
+1 if A’s numeric value is positive,
0⁺ if A’s numeric value is 0⁺,
and 0⁻ if A’s numeric value is 0⁻.
The return type is a

made consistent
with the input
calculation’s
type.
Note:
Both of these functions operate on the fully simplified/resolved form of their arguments,
which may give unintuitive results at first glance.
In particular,
an expression like
10%
might be positive
or
negative once it’s resolved,
depending on what value it’s resolved against.
For example, in
background-position
positive percentages
resolve to a negative length,
and vice versa,
if the background image is larger than the background area.
Thus
sign(10%)
might return
or
-1
depending on how the percentage is resolved!
(Or even
, if it’s resolved against a zero length.)
10.7.
Numeric Keywords
Keywords in
calculations
provide access to values that are difficult or impossible to represent as literals.
Each keyword defines its value,
its
type
and when it can be resolved.
10.7.1.
Numeric Constants:
pi
While the trigonometric and exponential functions
handle many complex numeric operations,
some reasonable calculations must be put together more manually,
and many times these include well-known constants,
such as
and
Rather than require authors to manually type out
several digits of these constants,
a few of them are provided directly:
the base of the natural logarithm,
approximately equal to 2.7182818284590452354.
pi
the ratio of a circle’s circumference to its diameter,
approximately equal to 3.1415926535897932.
Both of these keywords are

s,
and resolve at parse time.
Note:
These keywords are only usable within a calculation,
such as
calc(pow(e, pi) - pi)
, or
min(pi, 5, e)
If used outside of a calculation,
they’re treated like any other keyword:
animation-name: pi;
refers to an animation named "pi";
line-height: e;
is invalid
not
similar to
line-height: 2.7
but
line-height: calc(e);
is).
10.7.2.
Degenerate Numeric Constants:
infinity
-infinity
NaN
When a
calculation
or a subtree of a
calculation
becomes
infinite
or
NaN
representing it with a numeric value is no longer possible.
A UA may have an
implementation-defined limit for values approaching infinity
To aid in serialization of these degenerate values,
the following additional math constants are defined:
infinity
the value positive infinity (+∞)
-infinity
the value negative infinity (−∞)
NaN
the value NaN
All of these keywords are

s,
and resolve at parse time.
As usual for CSS keywords,
these are
ASCII case-insensitive
Thus,
calc(InFiNiTy)
is perfectly valid.
However,
NaN
must be serialized with this canonical casing.
Note:
As these keywords are

s,
to get an infinite length,
for example,
requires an expression like
calc(infinity * 1px)
Note:
These constants are defined
mostly
to make serialization of infinite/NaN values simpler and more obvious,
but
can
be used to indicate a "largest possible value",
since an infinite value gets clamped to the allowed range.
It’s rare for this to be reasonable,
but when it is,
using
infinity
is clearer in its intent
than just putting an enormous number in one’s stylesheet.
10.7.3.
Numeric Variables
Other specifications can define additional keywords
which are usable in
calculations
in certain contexts.
For example,
relative color
syntax
defines a number of color-channel keywords
representing the value of each color channel as a

Each specifications defining such keywords
must define for each keyword:
its value
its
type


, etc)
when it resolves (parse time, computed-value time, or used-value time)
10.8.
Syntax
The syntax of a
math function
is:
calc
()
calc

min
()
min

max
()
max

clamp
()
clamp

none


none
round
()
round



mod
()
mod


rem
()
rem


sin
()
sin

cos
()
cos

tan
()
tan

asin
()
asin

acos
()
acos

atan
()
atan

atan2
()
atan2


pow
()
pow


sqrt
()
sqrt

hypot
()
hypot

log
()
log


exp
()
exp

abs
()
abs

sign
()
sign



'+'
'-'



'*'








= e
pi
infinity
-infinity
NaN

= nearest
up
down
to-zero
line-width
In some contexts, additional

values
can be defined to be valid.
(For example, in
relative color
syntax,
appropriate channel keywords are allowed.)
In addition,
whitespace
is required on both sides of the
and
operators.
(The
and
operators can be used without white space around them.)
Several of the math functions above have additional constraints
on what their

arguments can contain.
Check the definitions of the individual functions for details.
UAs must support
calculations
of at least 32

and at least 32 levels of nesting (parentheses and/or functions).
For functions that support an arbitrary number of arguments (such as
min()
),
it must also support at least 32 arguments.
If a
calculation
contains more than the supported number of terms, arguments, or nesting
it must be treated as if it were invalid.
10.9.
Type Checking
math function
can be many possible types,
such as


, etc.,
depending on the
calculations
it contains,
as defined below.
It can be used anywhere a value of that type is allowed.
For example, the
width
property accepts

values,
so a
math function
that resolves to a

such as
calc(5px + 1em)
can be used in
width
Additionally,
math functions
that resolve to

can be used in any place that only accepts

the value is
rounded to the nearest integer
as it resolves.
Operators form sub-expressions, which gain types based on their arguments.
Note:
In previous versions of this specification,
multiplication and division were limited in what arguments they could take,
to avoid producing more complex intermediate results
(such as
1px * 1em
, which is

²)
and to make division-by-zero detectable at parse time.
This version now relaxes those restrictions.
To
determine the type of a
calculation
At a
or
sub-expression,
attempt to
add the types
of the left and right arguments.
If this returns failure,
the entire
calculation’s
type is failure.
Otherwise, the sub-expression’s
type
is the returned type.
At a
sub-expression,
multiply the types
of the left and right arguments.
The sub-expression’s
type
is the returned result.
At a
sub-expression,
let
left type
be the result of finding the
types
of its left argument,
and
right type
be the result of finding the
types
of its right argument
and then
inverting
it.
The sub-expression’s
type
is the result of
multiplying
the
left type
and
right type
Anything else is a terminal value,
whose
type
is determined based on its CSS type.
(Unless otherwise specified,
the type’s associated
percent hint
is null.)


the
type
is «[ ]» (empty map)

the
type
is «[ "length" → 1 ]»

the
type
is «[ "angle" → 1 ]»

the
type
is «[ "time" → 1 ]»

the
type
is «[ "frequency" → 1 ]»

the
type
is «[ "resolution" → 1 ]»

the
type
is «[ "flex" → 1 ]»

the
type
is as defined by the keyword

If, in the context in which the
math function
containing this
calculation
is placed,

s are resolved relative to another type of value
(such as in
width
, where

is resolved against a

),
and that other type is
not

the
type
is determined as the other type,
but with a
percent hint
set to that other type.
Otherwise,
the
type
is «[ "percent" → 1 ]»,
with a
percent hint
of "percent".
anything else
The
calculation’s
type is failure.
A value
contains a percentage
if its
type
is «[ "percent" → 1 ]»,
or its type’s
percent hint
is non-null.
Two or more calculations have a
consistent type
if
adding the types
doesn’t result in failure.
The
consistent type
is the result of the type addition.
To
make a type
base
consistent
with another type
input
If both
base
and
input
have different non-null
percent hints
they can’t be made consistent.
Return failure.
If
base
has a null
percent hint
set
base
’s
percent hint
to
input
’s
percent hint
Return
base
Math functions
themselves have
types
according to their contained
calculations
calc()
abs()
The
type
of its contained
calculation
min()
max()
clamp()
hypot()
round()
mod()
rem()
The result of
adding the types
of its comma-separated
calculations
asin()
acos()
atan()
atan2()
«[ "angle" → 1 ]».
sign()
sin()
cos()
tan()
pow()
sqrt()
log()
exp()
«[ ]» (empty map).
For each of the above,
if the
type
is failure,
the
math function
is invalid.
math function
resolves to







, or

according to which of those productions its
type
matches
(These categories are mutually exclusive.)
If it can’t
match
any of these,
the
math function
is invalid.
Note:
Algebraic simplifications do not affect the validity of a
math function
or its resolved type.
For example,
calc(5px - 5px + 10s)
and
calc(0 * 5px + 10s)
are both invalid
due to the attempt to add a length and a time.
Note:
Note that

s relative to

s,
such as in
opacity
are not
combinable
with those numbers—​
opacity: calc(.25 + 25%)
is invalid.
Allowing this causes significant problems with "unit algebra"
(allowing multiplication/division of

s),
and in every case so far,
doesn’t provide any new functionality.
(For example,
opacity: 25%
is identical to
opacity: .25
it’s just a trivial syntax transform.)
You can still perform other operations with them,
such as
opacity: calc(100% / 3);
which is valid.
Note:
Because

s are always interpreted as

s or

s,
"unitless 0"

s aren’t supported in
math functions
That is,
width: calc(0 + 5px);
is invalid,
because it’s trying to add a

to a

even though both
width: 0;
and
width: 5px;
are valid.
Note:
Although there are a few properties in which a bare

becomes a

at used-value time
(specifically,
line-height
and
tab-size
),

s never become "length-like" in
calc()
They always stay as

s.
Note:
In Quirks Mode
[QUIRKS]
some properties that would normally only accept

are defined to also accept

s,
interpreting them as
px
lengths.
Like unitless zeroes,
this has no effect on the parsing or behavior of
math functions
though a
math function
that resolves to a

value
might become valid in Quirks Mode
(and have its result interpreted as a
px
length).
10.9.1.
Calculation Contexts
Numeric values can be interpreted in various
calculation contexts
depending on where they are used,
which defines how

values resolve,
etc.
For example, in
top
, a

value
is resolved against the size of the containing block,
making it act as a

A single property can define multiple
calculation contexts
for example, in
background-size
one value resolves

s against the width of the
background positioning area
while the other resolves them against the height.
Math functions
always inherit the
calculation context
from wherever they’re used.
That is, in
top: calc(25% + 50px)
the

is resolved
as normal
for
top
as if
top: 25%
were specified
(and then has
50px
added to its value).
Unless otherwise defined,
all other functions
do not inherit a
calculation context
and instead define their own
calculation contexts
for their numeric arguments.
For example, in
top: anchor(25%)
the

in the
anchor()
function
is instead defined to resolve in a completely different manner
(against the height of the default anchor element).
10.9.2.
Infinities, NaN, and Signed Zero
Math functions
follow IEEE-754 semantics,
which means they recognize the concepts of
positive and negative zero,
positive and negative infinity,
and NaN (not a number).
However, these concepts are only retained within a
calculation tree
if a
top-level calculation
(a
math function
not nested directly inside of another
math function
would result in one of these special values,
they’re instead "censored" into a standard representable value,
as defined below.
Signed zeros
(indicated here as 0⁺ or 0⁻)
can not be written directly in CSS;
+0
and
-0
all produce the standard "unsigned" zero,
which is considered positive (0⁺)
for the purposes of these rules.
Signed zeroes
are produced in the following ways:
Negative zero
(0⁻)
can be produced by a multiplication or division that produces zero
with exactly one negative argument
(such as
-5 * 0
or
1 / -infinity
).
0⁻ + 0⁻
or
0⁻ - 0⁺
produces 0⁻.
All other additions or subtractions that would produce a zero
produce 0⁺.
Multiplying or dividing 0⁻ with a positive number
(including 0⁺)
produces a negative result
(either 0⁻ or −∞),
while multiplying or dividing 0⁻ with a negative number
produces a positive result.
(In other words,
multiplying or dividing with 0⁻
follows standard sign rules.)
When comparing 0⁺ and 0⁻,
0⁻ is less than 0⁺.
For example,
min(0⁺, 0⁻)
must produce 0⁻,
max(0⁺, 0⁻)
must produce 0⁺,
and
clamp(0⁺, 0⁻, 1)
must produce 0⁺.
Certain argument combinations in
math functions
are defined to produce 0⁻
(for example,
round(-1, infinity)
).
All other operations that produce a zero
produce positive zero (0⁺).
Signed zeroes
do not escape a
top-level calculation
they’re censored into the "unsigned" zero.
Infinities
(indicated here as +∞ or −∞)
can be written directly
using the
math constants
infinity
and
-infinity
or produced as a result of some calculations:
Dividing a value by zero produces either +∞ or −∞,
according to the standard sign rules.
Adding or subtracting ±∞ to anything produces the appropriate infinity.
Multiplying any value by ±∞ produces the appropriate infinity.
Dividing any value by ±∞ produces zero.
Certain argument combinations in
math functions
are defined to produce
infinities
(for example,
pow(0, -1)
produces +∞).
Note:
The rules for producing
NaN
, below,
supersede the above rules for producing
infinities
Infinities
do not escape a
top-level calculation
they’re clamped to the minimum or maximum value allowed in the context,
as defined in
§ 10.12 Range Checking
NaN
(short for "not a number")
is the result of certain operations
that don’t have a well-defined value.
It can be written directly
using the
math constants
NaN
or produced as a result of some calculations:
Dividing zero by zero,
dividing ±∞ by ±∞,
multiplying 0 by ±∞,
adding +∞ to −∞,
or subtracting two infinities of the same sign
produces NaN.
These rules override any other result,
if there’s a conflict.
For example,
0 / 0
is NaN, not +∞.
Certain argument combinations in
math functions
are defined to produce
NaN
(for example,
asin(2)
produces NaN).
Any operation with at least one NaN argument produces NaN.
NaN
does not escape a
top-level calculation
it’s censored into a zero value
For example,
calc(-5 * 0)
produces an unsigned zero—​the calculation resolves to 0⁻,
but as it’s a
top-level calculation
it’s then censored to an unsigned zero.
On the other hand,
calc(1 / calc(-5 * 0))
produces −∞,
same as
calc(1 / (-5 * 0))
—​the inner calc resolves to 0⁻,
and as it’s not a
top-level calculation
it passes it up unchanged to the outer calc to produce −∞.
If it was censored into an unsigned zero,
it would instead produce +∞.
10.10.
Internal Representation
The
internal representation
of a
math function
is a
calculation tree
a tree where the branch nodes are
operator nodes
corresponding either to
math functions
(such as Min, Cos, Sqrt, etc)
or to operators in a
calculation
(Sum, Product, Negate, and Invert, the
calc-operator nodes
),
and the leaf nodes
are either numeric values
(such as numbers, dimensions, and percentages)
or non-
math functions
that resolve to a numeric type.
Math functions
are turned into
calculation trees
depending on the function:
calc()
The
internal representation
of a
calc()
function
is the result of
parsing a calculation
from its argument.
any other
math function
The
internal representation
is an
operator node
with the same name as the function,
whose children are the result of
parsing a calculation
from each of the function’s arguments,
in the order they appear.
To
parse a calculation
given a
calculation
values
represented as a list of
component values
and returning a
calculation tree
Discard any

s from
values
An item in
values
is an “operator”
if it’s a

with the value "+", "-", "*", or "/".
Otherwise, it’s a “value”.
Collect children into Product and Invert nodes.
For every consecutive run of value items in
values
separated by "*" or "/" operators:
For each "/" operator in the run,
replace its right-hand value item
rhs
with an Invert node containing
rhs
as its child.
Replace the entire run with a Product node
containing the value items of the run as its children.
Collect children into Sum and Negate nodes.
For each "-" operator item in
values
replace its right-hand value item
rhs
with a Negate node containing
rhs
as its child.
If
values
has only one item,
and it is a Product node
or a parenthesized
simple block
replace
values
with that item.
Otherwise,
replace
values
with a Sum node
containing the value items of
values
as its children.
At this point
values
is
a tree of Sum, Product, Negate, and Invert nodes,
with other types of values at the leaf nodes.
Process the leaf nodes.
For every leaf node
leaf
in
values
If
leaf
is a parenthesized
simple block
replace
leaf
with the result of
parsing a calculation
from
leaf
’s contents.
If
leaf
is a
math function
replace
leaf
with the
internal representation
of that math function.
Return the result of
simplifying a calculation tree
from
values
10.10.1.
Simplification
Internal representations
of
math functions
are eagerly simplified to the extent possible,
using standard algebraic simplifications
(distributing multiplication over sums,
combining similar units,
etc.).
When used in non-property contexts
(such as in
@font-face
descriptors, for example),
math functions
are simplified
as if they were
specified values
To
simplify a calculation tree
root
If
root
is a numeric value:
If
root
is a percentage that will be resolved against another value,
and there is enough information available to resolve it,
do so,
and express the resulting numeric value
in the appropriate
canonical unit
Return the value.
If
root
is a dimension
that is not expressed in its
canonical unit
and there is enough information available to convert it to the
canonical unit
do so,
and return the value.
If
root
is a

that can be resolved,
return what it resolves to,
simplified
Otherwise, return
root
If
root
is any other leaf node
(not an operator node):
If there is enough information available to determine its numeric value,
return its value,
expressed in the value’s
canonical unit
Otherwise, return
root
At this point,
root
is an
operator node
Simplify
all the
calculation
children of
root
If
root
is an
operator node
that’s not one of the
calc-operator nodes
and all of its
calculation
children are numeric values
with enough information to compute the operation
root
represents,
return the result of running
root
’s operation
using its children,
expressed in the result’s
canonical unit
If a percentage is left at this point,
it will
usually
block simplification of the node,
since it needs to be resolved against another value
using information not currently available.
(Otherwise, it would have been converted to a different value in an earlier step.)
This includes operations such as "min",
since percentages might resolve against a negative basis,
and thus end up with an opposite comparative relationship
than the raw percentage value would seem to indicate.
However, "raw" percentages—​ones which do not resolve against another value,
such as in
opacity
—​might not block simplification.
If
root
is a Min or Max node,
attempt to
partially
simplify it:
For each
node
child
of
root
’s children:
If
child
is a numeric value
with enough information to compare magnitudes with another child of the same unit
(see note in previous step),
and there are other children of
root
that are numeric values with the same unit,
combine all such children
with the appropriate operator per
root
and replace
child
with the result,
removing all other child nodes involved.
If
root
has only one child,
return the child.
Otherwise, return
root
If
root
is a Negate node:
If
root
’s child is a numeric value,
return an equivalent numeric value,
but with the value negated
(0 - value).
If
root
’s child is a Negate node,
return the child’s child.
Return
root
If
root
is an Invert node:
If
root
’s child is a number
(not a percentage or dimension)
return the reciprocal of the child’s value.
If
root
’s child is an Invert node,
return the child’s child.
Return
root
If
root
is a Sum node:
For each of
root
’s children
that are Sum nodes,
replace them with their children.
For each set of
root
’s children
that are numeric values with identical units,
remove those children
and replace them with a single numeric value
containing the sum of the removed nodes,
and with the same unit.
(E.g. combine numbers, combine percentages,
combine px values, etc.)
If
root
has only a single child at this point,
return the child.
Otherwise, return
root
Note:
Zero-valued terms cannot be simply removed from a Sum;
they can only be combined with other values that have identical units.
(This is because the mere presence of a unit, even with a zero value,
can sometimes imply a change in behavior.)
If
root
is a Product node:
For each of
root
’s children
that are Product nodes,
replace them with their children.
If
root
has multiple children that are numbers
(not percentages or dimensions),
remove them and replace them with a single number
containing the product of the removed nodes.
If
root
contains only two children,
one of which is a number
(not a percentage or dimension)
and the other of which is a Sum
whose children are all numeric values,
multiply all of the Sum’s children by the number,
then return the Sum.
If
root
contains only numeric values
and/or Invert nodes containing numeric values,
and
multiplying the types
of all the children
(noting that the type of an Invert node
is the
inverse
of its child’s type)
results in a type that
matches
any of the types that a
math function
can resolve to,
return the result of multiplying all the values of the children
(noting that the value of an Invert node
is the reciprocal of its child’s value),
expressed in the result’s
canonical unit
Return
root
Return
root
10.11.
Computed Value
The
computed value
of a
math function
is its
calculation tree
simplified
using all the information available at
computed value
time.
(Such as the
em
to
px
ratio,
how to resolve percentages in some properties,
etc.)
Where percentages are not resolved at computed-value time,
they are not resolved in
math functions
e.g.
calc(100% - 100% + 1px)
resolves to
calc(0% + 1px)
not to
1px
If there are special rules for computing percentages in a value
(e.g.
the
height
property
),
they apply whenever a
math function
contains percentages.
The
calculation tree
is again simplified at
used value
time;
with
used value
time information,
math function
always simplifies down to a single numeric value.
For example,
whereas
font-size
computes percentage values at
computed value
time
so that
font-relative length
units can be computed,
background-position
has layout-dependent behavior for percentage values,
and thus does not resolve percentages until used-value time.
Due to this,
background-position
computation preserves the percentage in a
calc()
whereas
font-size
will compute such expressions directly into a length.
Given the complexities of width and height calculations on table cells and table elements,
math expressions mixing both percentages and non-zero lengths for widths and heights on
table columns, table column groups, table rows, table row groups, and table cells
in both auto and fixed layout tables
MUST be treated as if
auto
had been specified.
10.12.
Range Checking
Parse-time range-checking of values is not performed within
math functions
and therefore out-of-range values do not cause the declaration to become invalid.
However, the value resulting from a
top-level calculation
must be clamped to the range allowed in the target context.
Clamping is performed on
computed values
to the extent possible,
and also on
used values
if computation was unable to sufficiently simplify the expression
to allow range-checking.
(Clamping is not performed on
specified values
.)
Note:
This requires all contexts accepting
calc()
to define their allowable values as a closed (not open) interval.
Note:
By definition,
±∞ are outside the allowed range for any property,
and will clamp to the minimum/maximum value allowed.
Even properties that can explicitly represent infinity as a keyword value,
such as
animation-iteration-count
will end up clamping ±∞,
as
math functions
can’t resolve to keyword values;
the
numeric
part of the property’s syntax still has a minimum/maximum value.
Additionally, if a
math function
that resolves to

is used somewhere that only accepts

the
computed value
and
used value
are
rounded to the nearest integer
in the same manner as clamping, above.
Since widths smaller than 0px are not allowed,
these three declarations are equivalent:
width
calc
px
10
px
);
width
calc
-5
px
);
width
px
Note however that
width: -5px
is not equivalent to
width: calc(-5px)
Out-of-range values
outside
calc()
are syntactically invalid,
and cause the entire declaration to be dropped.
Tests
calc-integer.html
(live test)
(source)
calc-z-index-fractions-001.html
(live test)
(source)
10.13.
Serialization
To
serialize a math function
fn
If the root of the
calculation tree
fn
represents
is a numeric value
(number, percentage, or dimension),
and the serialization being produced is of a
computed value
or later,
then clamp the value to the range allowed for its context
(if necessary),
then serialize the value as normal
and return the result.
If
fn
represents an infinite or NaN value:
Let
be the
string
"calc(".
Serialize the keyword
infinity
-infinity
, or
NaN
as appropriate to represent the value,
and append it to
If
fn
’s
type
is anything other than «[ ]»
(empty, representing a

),
append " * " to
Create a numeric value
in the
canonical unit
for
fn
’s
type
(such as
px
for

),
with a value of 1.
Serialize this numeric value
and append it to
Append ")" to
, then return it.
If the
calculation tree’s
root node is a numeric value,
or a
calc-operator node
let
be a string initially containing "calc(".
Otherwise,
let
be a string initially containing the name of the root node,
lowercased
(such as "sin" or "max"),
followed by a "(" (open parenthesis).
For each child of the root node,
serialize the calculation tree
If a result of this serialization starts with a "(" (open parenthesis)
and ends with a ")" (close parenthesis),
remove those characters from the result.
Concatenate
all of the results
using ", " (comma followed by space),
then append the result to
Append ")" (close parenthesis) to
Return
To
serialize a calculation tree
Let
root
be the root node
of the
calculation tree
If
root
is a numeric value,
or a non-
math function
serialize
root
per the normal rules for it
and return the result.
If
root
is anything but
a Sum,
Negate,
Product,
or Invert node,
serialize a math function
for the function corresponding to the node type,
treating the node’s children as the function’s comma-separated
calculation
arguments,
and return the result.
If
root
is a Negate node:
Let
be a
string
initially containing "(-1 * ".
Serialize
root
’s child,
and append it to
Append ")" to
then return it.
If
root
is an Invert node:
Let
be a
string
initially containing "(1 / ".
Serialize
root
’s child,
and append it to
Append ")" to
then return it.
If
root
is a Sum node:
Let
be a
string
initially containing "(".
Sort root’s children
Serialize
root
’s first child,
and append it to
For each
child
of
root
beyond the first:
If
child
is a Negate node,
append " - " to
then
serialize
the Negate’s child
and append the result to
If
child
is a negative numeric value,
append " - " to
then serialize the negation of
child
as normal
and append the result to
Otherwise,
append " + " to
then
serialize
child
and append the result to
Append ")" to
and return it.
If
root
is a Product node:
Let
be a
string
initially containing "(".
Sort root’s children
Serialize
root
’s first child,
and append it to
For each
child
of
root
beyond the first:
If
child
is an Invert node,
append " / " to
then
serialize
the Invert’s child
and append the result to
Otherwise,
append " * " to
then
serialize
child
and append the result to
Append ")" to
and return it.
To
sort a calculation’s children
nodes
Let
ret
be an empty list.
If
nodes
contains a number,
remove it from
nodes
and append it to
ret
If
nodes
contains a percentage,
remove it from
nodes
and append it to
ret
If
nodes
contains any dimensions,
remove them from
nodes
sort them by their units,
ordered
ASCII case-insensitively
and append them to
ret
If
nodes
still contains any items,
append them to
ret
in the same order.
Return
ret
Tests
calc-rgb-percent-001.html
(live test)
(source)
calc-serialization.html
(live test)
(source)
calc-serialization-002.html
(live test)
(source)
getComputedStyle-border-radius-001.html
(live test)
(source)
getComputedStyle-border-radius-003.html
(live test)
(source)
calc-background-position-003.html
(live test)
(source)
calc-nesting-002.html
(live test)
(source)
For example,
calc(20px + 30px)
would serialize as
calc(50px)
as a specified value,
or as
50px
as a computed value.
A value like
calc(20px + 0%)
would serialize as
calc(0% + 20px)
maintaining both terms in the serialized value.
(It’s important to maintain zero-valued terms,
so the
calc()
doesn’t suddenly "change shape" in the middle of a transition
when one of the values happens to have a zero value temporarily.
This also removes the need to "pick a unit" when all the terms are zero.)
A value like
calc(20px + 2em)
would serialize as
calc(2em + 20px)
as a specified value
(maintaining both units as they’re incompatible at specified-value time,
but sorting them alphabetically),
or as something like
52px
as a computed value
em
values are converted to absolute lengths at computed-value time,
so assuming
1em
16px
they combine into
52px
which then drops the
calc()
wrapper.)
When used in non-property contexts
(such as in
@font-face
descriptors, for example),
math functions
are simplified
as if they were
specified values
See
[CSSOM]
for further information on serialization.
10.14.
Combination of Math Functions
Interpolation
of
math functions
with each other
or with numeric values and other numeric-valued functions,
is defined as
result
= calc((1 - p) * V
+ p * V
).
Simplification
of the value might then reduce the expression
to a smaller, simpler form.)
Addition
of
math functions
with each other
or with numeric values and other numeric-valued functions,
is defined as
result
= calc(V
+ V
).
Simplification
of the value might then reduce the expression
to a smaller, simpler form.)
Appendix A: Coordinating List-Valued Properties
Some list-valued properties have coordinated effects:
each item in their value list applies to a distinct effect,
and corresponding entries in each property’s list
all refer to the same effect.
Often the coordinating values can also be specified together
as a single entry in a list-valued
shorthand property
A typical example is the list-valued
background-*
properties,
which can specify
multiple background image layers
For each property controlling how the image is sized, tiled, placed, etc.,
the
th item in its list describes some effect that applies
to the
th background image.
coordinating list property group
creates
coordinated value list
which has, for each entry,
a value from each property in the group;
these are used together to define a single effect,
such as a background image layer
or an animation.
The
coordinated value list
is assembled as follows:
The length of the
coordinated value list
is determined by
the number of items specified in one particular
coordinating list property
the
coordinating list base property
(In the case of backgrounds, this is the
background-image
property.)
The
th value of the
coordinated value list
is constructed
by collecting the
th
use value
of each
coordinating list property
If a
coordinating list property
has too many values specified,
excess values at the end of its list are not
used
If a
coordinating list property
has too few values specified,
its value list is repeated to add more
used values
The
computed values
of the
coordinating list properties
are not affected by such truncation or repetition.
Appendix B: IANA Considerations
Registration for the
about
invalid
URL scheme
This sections defines and registers the
about
invalid
URL,
in accordance with the registration procedure defined in
[RFC6694]
The official record of this registration can be found at
Registered Token
invalid
Intended Usage
The
about
invalid
URL references a non-existent document with a generic error condition.
It can be used when a URL is necessary, but the default value shouldn’t be resolvable as any type of document.
Contact/Change controller
CSS WG <
www-style@w3.org
> (on behalf of W3C)
Specification
CSS Values and Units Module Level 3
Appendix C: Quirky Lengths
When CSS is being parsed in
quirks mode

is a type of

that is only valid in certain properties:
background-position
border-spacing
border-top-width
border-right-width
border-bottom-width
border-left-width
border-width
bottom
clip
font-size
height
left
letter-spacing
margin-right
margin-left
margin-top
margin-bottom
margin
max-height
max-width
min-height
min-width
padding-top
padding-right
padding-bottom
padding-left
padding
right
text-indent
top
vertical-align
width
word-spacing
It is
not
valid in properties that include or reference these properties,
such as the
background
shorthand,
or inside
functional notations
such as
calc()
except that they must be allowed in
rect()
in the
clip
property.
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.

is syntactically identical to a

and is interpreted as a
px
length with the same value.
(In other words, Quirks Mode allows all
px
lengths in the affected properties
to be written without a unit,
similar to unitless zero lengths.)
Acknowledgments
Firstly, the editors would like to thank
all of the contributors to the
previous level
of this module.
Secondly, we would like to acknowledge
Anthony Frehner,
Emilio Cobos Álvarez,
Guillaume Lebas,
Koji Ishii,
Noam Rosenthal,
and
Xidorn Quan
for their comments and suggestions,
which have improved Level 4.
Changes
Recent Changes
(This is a subset of
Additions Since Level 3
.)
Substantial changes since
18 December 2023 WD
Added the
none
values to
clamp()
Issue 9713
Generally fixed how type inference handles percentages.
Issue 10017
Restored dependency of
viewport-percentage lengths
on
overflow: scroll
and added one on
scrollbar-gutter
to make it possible for 100 of these units to actually match the
initial containing block
Issue 6026
Allow B to be omitted in
round()
if the A’s type is

Issue 9668
Substantial changes since
27 October 2023 WD
Pinned the
default viewport-percentage units
to the
large viewport-percentage units
—​despite violation of the “avoid dataloss by default principle”—​given existing interoperability and presumed Web-compat restriction.
Issue 6452
Added an explicit definition for the
CSS grammar production block
convention.
Issue 2921
Clarified character encoding of percent-encoded URLs.
Issue 9301
Substantial changes since
6 April 2023 WD
Punted
mix()
to
[css-values-5]
Issue 9343
Color type defined to be non-additive.
Issue 8576
Non-property contexts treat
math functions
as specified values.
Issue 7964
Specified that URLs from CSS are always transmitted as UTF-8.
Issue 9301
Fixed
addition
accumulation
to use the *second* value,
when the two values aren’t additive/accumulative.
Issue 9070
Specified that
font-relative lengths
are always resolved against the parent element
when used in a
font-*
property.
Issue 8169
Simplify away single-argument
min()
and
max()
functions.
Issue 9559
Substantial changes since
19 October 2022 WD
Added
§ 2.6 Functional Notation Definitions
to formally define the way that
functional notation
syntaxes are defined.
Issue 2921
Added algorithm for
snap as a border width
to reflect the interoperable rules for rendering consistent stroke widths.
Issue 5210
Clarified grammar and
computed value
of
mix()
Issue 8096
Undefined the behavior of
tan()
at the asymptote values.
Issue 8527
Specified that negative

values are out-of-range by definition.
Issue 8532
Clarified that fully omitted
mix()
arguments are valid.
Issue 8556
Clarified that range clamping happens specifically to
top-level calculations
(rather than the unclear term "expressions").
Issue 8158
Added formal definitions for

and

(in addition to

).
Rephrased fragment-only URLs in terms of tree-scoped references.
Issue 3320
Substantial changes since
16 December 2021 WD
Changed resolution of a
url()
with the
local url flag
to reference the current
node tree
(regardless of document base URL modifications).
Issue 3320
Switched censoring of
NaN
that escapes a
math function
from infinity to zero.
Issue 7067
Added
Appendix A: Coordinating List-Valued Properties
to allow this property pattern to be easily referenced.
Issue 7164
Restricted
mix()
to be the sole value of a declaration.
Issue 6700
Updated to match latest Fetch terminology.
Fetch PR 1413
CSS PR 7160
Clarified that the
font-relative lengths
are calculated without text shaping.
Defined serialization of empty urls to be
url
""
Issue 6447
Defined serialization of

specified values
Issue 2274
Fixed definition of
numbers
to allow decimals in combination with scientific notation,
as originally intended and as defined in
[CSS-SYNTAX-3]
Issue 7248
Corrected various functions to return an empty map
for their
type
instead of «[ "number" → 1 ]».
Issue 7486
Clarified effect of special UA restrictions on
line-height
on
lh
and
rlh
Issue 3257
Defined
function
()
notation to refer to functional notations.
Issue 5728
Substantial changes since
16 October 2021 WD
Switched
*vi
and
*vb
units to resolve against the computed
writing mode
of the element itself.
Issue 6873
Added
§ 4.5.4 URL Processing Model
to define integration with CORS, etc.
Issue 562
Fixed the inverted assignment of
viewport-percentage length
behaviors
to types of interface changes (A vs. B).
Changes in interface that happen as a result of scrolling
or other frequent page interactions that would disturb the user
if they resulted in substantial layout changes
must be categorized as the
former (A)
latter (B)
Changes in interface that have a sufficiently steady state
that re-laying out the document into the adjusted space
would be beneficial to the user
must be categorized as the
latter (B)
former (A)
Defined minimum number of
calc()
terms, arguments, and nesting as 32.
Issue 3462
Defined that
mod(-0, infinity)
returns
NaN
Issue 4723
Deferred
toggle()
and
attr()
to Level 5.
Changes since
30 September 2021 WD
Added
rex
rcap
rch
, and
ric
units.
Switched
toggle()
to use semicolons, matching with
mix()
Issue 6701
Fixed some wording errors in the definition of
calc()
Issue 6506
Imported definition of

from
[QUIRKS]
Issue 6100
Changes since
7 July 2021 WD
Added
mix()
notation for representing interpolated values.
Defined generically the computation of



, and

Clarified that only non-zero lengths create a percentage+length mix
that switches table cells to
auto
sizing.
Changes since
11 November 2020 WD
Updated interpolation of colors to reference
[CSS-COLOR-4]
instead of
[CSS-COLOR-3]
Added the
svh
svw
svi
svb
svmin
, and
svmax
small viewport-percentage units
lvh
lvw
lvi
lvb
lvmin
, and
lvmax
large viewport-percentage units
and
dvh
dvw
dvi
dvb
dvmin
, and
dvmax
dynamic viewport-percentage units
Issue 4329
and
Issue 6113
Clamped excessively large

values to multiples of
360deg
Issue 6105
Added back
rules on range-checking combined values
lost during move from the
CSS Transitions
specification.
Issue 6097
Specified that UA-imposed minimum font sizes apply to the used
font-size
and not to resolution of
font-relative lengths
Issue 5858
Clarified how
min()
and
max()
percentages can partially simplify.
Issue 6293
Additions Since Level 3
Changes since
CSS Values and Units Level 3
Explicitly undefined numeric precision/range.
Added rules for interpolation per value type,
and their clarified computed values.
Updated interpolation of colors to reference
[CSS-COLOR-4]
Additions since
CSS Values and Units Level 3
Defined the

type.
Defined the

type.
Added
src()
to the

type.
Added the
vi
vb
ic
cap
lh
and
rlh
length units.
Added the
svh
svw
svi
svb
svmin
, and
svmax
small viewport-percentage units
and
dvh
dvw
dvi
dvb
dvmin
, and
dvmax
dynamic viewport-percentage units
Added the
alias to
dppx
Added
min()
max()
, and
clamp()
comparison functions
Added
round()
mod()
rem()
sin()
cos()
tan()
asin()
acos()
atan()
atan2()
pow()
sqrt()
hypot()
log()
exp()
abs()
sign()
math functions.
Added
pi
infinity
-infinity
NaN
constants for use in
calc()
Added
unit algebra
to
calc()
allowing multiplication and division of
dimensions
A non-integer in a calc() automatically rounds to the nearest integer
when used where an

is required.
Defined
serialization
of
math functions
Added a genericized definition of
coordinating list property groups
to make it easier to reference the coordinating behavior of the
background
properties.
Security Considerations
This specification presents no new security considerations.
This specification defines the
url()
and
src()
functions (

),
which allow CSS to make network requests.
Depending on what features they are used in,
these can potentially expose whether or not the user has access to resources on a network,
and expose information about their contents
(such as the rules within a style sheet, the size of an image, the metrics of a font).
They can also allow exfiltrating data via URL.
Privacy Considerations
This specification defines units that expose the user’s screen size
(the
viewport-percentage lengths
),
default font size,
and potentially some information about
which fonts are available on the user’s system
(the
font-relative lengths
).
This specification defines the
url()
and
src()
functions (

),
which allow CSS to make network requests.
Depending on what features they are used in,
these can potentially expose whether or not the user has access to resources on a network,
and expose information about their contents
(such as the rules within a style sheet, the size of an image, the metrics of a font).
They can also allow exfiltrating data via URL.
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.
Index
Terms defined by this specification
, in § 2.3
, in § 2.3
&&
, in § 2.2
, in § 2.3
, in § 2.3
, in § 2.1
, in § 2.3
, in § 2.2
||
, in § 2.2
{A}
, in § 2.3
{A,B}
, in § 2.3
abs()
, in § 10.6
absolute length
, in § 6.2
absolute length unit
, in § 6.2
accumulate
, in § 3
accumulation
, in § 3
accumulation procedure
, in § 3
acos()
, in § 10.4
add
, in § 3
addition
, in § 3
addition procedure
, in § 3
advance measure
, in § 6.1.1
anchor
, in § 6.2
anchor unit
, in § 6.2

, in § 7.1

, in § 5.6
asin()
, in § 10.4
atan()
, in § 10.4
atan2()
, in § 10.4
bearing angle
, in § 7.1
between zero and B
, in § 10.3
bracketed range notation
, in § 5.1
calc()
, in § 10.1

, in § 10.8
calc-operator nodes
, in § 10.10

, in § 10.8

, in § 10.8
calculation
, in § 10.1
calculation contexts
, in § 10.9.1
calculation tree
, in § 10.10

, in § 10.8
canonical
, in § 5.4.1
canonical unit
, in § 5.4.1
cap
, in § 6.1.1
cap unit
, in § 6.1.1
ch
, in § 6.1.1
ch unit
, in § 6.1.1
clamp()
, in § 10.2
cm
, in § 6.2
combine
, in § 3
compatible
, in § 5.4.1
compatible units
, in § 5.4.1
computed length
, in § 6
consistent
, in § 10.9
consistent type
, in § 10.9
contain a percentage
, in § 10.9
coordinated value list
, in § Unnumbered section
coordinating list base property
, in § Unnumbered section
coordinating list property
, in § Unnumbered section
coordinating list property group
, in § Unnumbered section
cos()
, in § 10.4
CSS bracketed range notation
, in § 5.1
CSS grammar production block
, in § 2.8
CSS ident
, in § 4
CSS identifier
, in § 4
CSS value definition syntax
, in § 2
CSS-wide keywords
, in § 4.1.1

, in § 4.2

, in § 4.3
default viewport-percentage units
, in § 6.1.2.1
deg
, in § 7.1
degenerate ratio
, in § 5.7
determine the type of a calculation
, in § 10.9
device pixel
, in § 6.2

, in § 5.4
dimension
, in § 5.4
down
, in § 10.3
dpcm
, in § 7.4
dpi
, in § 7.4
dppx
, in § 7.4
dvb
, in § 6.1.2.2
dvh
, in § 6.1.2.2
dvi
, in § 6.1.2.2
dvmax
, in § 6.1.2.2
dvmin
, in § 6.1.2.2
dvw
, in § 6.1.2.2
dynamic viewport-percentage units
, in § 6.1.2.1
dynamic viewport size
, in § 6.1.2.1
, in § 10.7.1
em
, in § 6.1.1
em unit
, in § 6.1.1
ex
, in § 6.1.1
exp()
, in § 10.5
ex unit
, in § 6.1.1
fetch a style resource
, in § 4.5.4
font-relative lengths
, in § 6.1.1

, in § 7.3

, in § 5.6
functional notation
, in § 9
grad
, in § 7.1
hypot()
, in § 10.5
Hz
, in § 7.3
ic
, in § 6.1.1
ic unit
, in § 6.1.1

, in § 4
ident
, in § 4
identifier
, in § 4
implementation-defined limit for values approaching infinity
, in § 10.7.2
in
, in § 6.2
infinite
, in § 10.9.2
-infinity
, in § 10.7.2
infinity
dfn for CSS
, in § 10.9.2
value for calc()
, in § 10.7.2

, in § 5.2
integer
, in § 5.2
interpolate
, in § 3
interpolation
, in § 3
interpolation procedure
, in § 3
keyword
, in § 4.1
kHz
, in § 7.3
large viewport-percentage units
, in § 6.1.2.1
large viewport size
, in § 6.1.2.1

, in § 6

, in § 5.6
lh
, in § 6.1.1
lh unit
, in § 6.1.1
line-width
, in § 10.3
local font-relative lengths
, in § 6.1.1
local url flag
, in § 4.5.1.1
log()
, in § 10.5
lvb
, in § 6.1.2.2
lvh
, in § 6.1.2.2
lvi
, in § 6.1.2.2
lvmax
, in § 6.1.2.2
lvmin
, in § 6.1.2.2
lvw
, in § 6.1.2.2
made consistent
, in § 10.9
make a type consistent
, in § 10.9
make consistent
, in § 10.9
math function
, in § 10
max()
, in § 10.2
min()
, in § 10.2
mm
, in § 6.2
mod()
, in § 10.3
ms
, in § 7.2
NaN
dfn for CSS
, in § 10.9.2
value for calc()
, in § 10.7.2
nearest
, in § 10.3
negative infinity
, in § 10.9.2
negative zero
, in § 10.9.2
none
, in § 10.2
not additive
, in § 3

, in § 5.3
number
, in § 5.3
numeric data types
, in § 5
operator nodes
, in § 10.10
parse a calculation
, in § 10.10
parsing a calculation
, in § 10.10
pc
, in § 6.2

, in § 5.5
percentage
, in § 5.5
percentage-containing
, in § 10.9
physical unit
, in § 6.2
pi
, in § 10.7.1
pixel unit
, in § 6.2

, in § 8.3
positive infinity
, in § 10.9.2
positive zero
, in § 10.9.2
pow()
, in § 10.5
pt
, in § 6.2
px
, in § 6.2
, in § 6.2

, in § Unnumbered section
rad
, in § 7.1

, in § 5.7
ratio
, in § 5.7
rcap
, in § 6.1.1
rcap unit
, in § 6.1.1
rch
, in § 6.1.1
rch unit
, in § 6.1.1
reference pixel
, in § 6.2
relative length
, in § 6.1
relative length unit
, in § 6.1
rem
, in § 6.1.1
rem()
, in § 10.3
rem unit
, in § 6.1.1

, in § 7.4
resolve a style resource URL
, in § 4.5.4
rex
, in § 6.1.1
rex unit
, in § 6.1.1
ric
, in § 6.1.1
ric unit
, in § 6.1.1
rlh
, in § 6.1.1
rlh unit
, in § 6.1.1
root font-relative lengths
, in § 6.1.1
round()
, in § 10.3

, in § 10.3
round to the nearest integer
, in § 5.2
, in § 7.2
serialize a calculation tree
, in § 10.13
serialize a math function
, in § 10.13
serialize the calculation tree
, in § 10.13
serializing a calculation tree
, in § 10.13
serializing the calculation tree
, in § 10.13
sign()
, in § 10.6
signed zero
, in § 10.9.2
simplify
, in § 10.10.1
simplify a calculation tree
, in § 10.10.1
simplifying a calculation tree
, in § 10.10.1
sin()
, in § 10.4
small viewport-percentage units
, in § 6.1.2.1
small viewport size
, in § 6.1.2.1
snap a length as a border width
, in § 6
snap as a border width
, in § 6
sort a calculation’s children
, in § 10.13
specified length
, in § 6
sqrt()
, in § 10.5
src()
, in § 4.5

, in § 4.4
style resource base URL
, in § 4.5.4
svb
, in § 6.1.2.2
svh
, in § 6.1.2.2
svi
, in § 6.1.2.2
svmax
, in § 6.1.2.2
svmin
, in § 6.1.2.2
svw
, in § 6.1.2.2
tan()
, in § 10.4
textual data types
, in § 4

, in § 7.2

, in § 5.6
top-level calculation
, in § 10.9.2
to-zero
, in § 10.3
turn
, in § 7.1
up
, in § 10.3

, in § 4.5
url()
, in § 4.5

, in § 4.5.3
URL request modifier steps
, in § 4.5.4
value accumulation
, in § 3
value addition
, in § 3
value definition syntax
, in § 2
value interpolation
, in § 3
vb
, in § 6.1.2.2
vh
, in § 6.1.2.2
vi
, in § 6.1.2.2
viewport-percentage lengths
, in § 6.1.2
visual angle unit
, in § 6.2
vmax
, in § 6.1.2.2
vmin
, in § 6.1.2.2
vw
, in § 6.1.2.2
, in § 7.4

, in § 5.3
Terms defined by reference
[CSS-2023]
defines the following terms:
UA
[CSS-ANCHOR-POSITION-1]
defines the following terms:
anchor()
[CSS-ANIMATIONS-1]
defines the following terms:
animation
animation-iteration-count
animation-name
animation-timing-function
[CSS-BORDERS-4]
defines the following terms:
border-bottom-width
border-color
border-left-width
border-right-width
border-top-width
border-width
box-shadow
[CSS-BOX-4]
defines the following terms:
margin
margin-bottom
margin-left
margin-right
margin-top
padding
padding-bottom
padding-left
padding-right
padding-top
[CSS-BREAK-3]
defines the following terms:
orphans
[CSS-CASCADE-5]
defines the following terms:
actual value
computed value
inherit
initial
shorthand property
specified value
unset
used value
[CSS-CASCADE-6]
defines the following terms:
@import
[CSS-COLOR-4]
defines the following terms:
hsl()
opacity
rgba()
[CSS-COLOR-5]
defines the following terms:

@color-profile
relative color
[CSS-CONDITIONAL-3]
defines the following terms:
@supports
supports(conditionText)
[CSS-COUNTER-STYLES-3]
defines the following terms:
disc
[CSS-DISPLAY-4]
defines the following terms:
containing block
initial containing block
root element
[CSS-EASING-2]
defines the following terms:

ease-in
ease-out
easing function
timing function
[CSS-FONTS-4]
defines the following terms:
font
font-family
font-size
[CSS-FONTS-5]
defines the following terms:
@font-face
font-size
[CSS-GRID-2]
defines the following terms:

fr
[CSS-IMAGES-4]
defines the following terms:
image-resolution
[CSS-INLINE-3]
defines the following terms:
normal
vertical-align
[CSS-MASKING-1]
defines the following terms:
clip
[CSS-OVERFLOW-3]
defines the following terms:
overflow
scrollbar-gutter
[CSS-OVERFLOW-4]
defines the following terms:
max-lines
[CSS-PAGE-3]
defines the following terms:
page area
[CSS-POSITION-3]
defines the following terms:
bottom
left
right
top
[CSS-RHYTHM-1]
defines the following terms:
block-step-size
[CSS-SHADOW-1]
defines the following terms:
tree-scoped name
tree-scoped reference
[CSS-SHAPES-1]
defines the following terms:
rect()
[CSS-SIZING-3]
defines the following terms:
auto
box-sizing
height
max-height
max-width
min-height
min-width
width
[CSS-SYNTAX-3]
defines the following terms:











at-rule
component value
consume a url token
escaping
simple block
whitespace
[CSS-TEXT-3]
defines the following terms:
text-align
[CSS-TEXT-4]
defines the following terms:
center
letter-spacing
tab-size
text-indent
word-spacing
[CSS-TEXT-DECOR-4]
defines the following terms:
text-decoration
[CSS-TRANSFORMS-1]
defines the following terms:
transform-origin
[CSS-TYPED-OM-1]
defines the following terms:
add two types
internal representation
invert a type
match
multiply two types
percent hint
type
[CSS-UI-4]
defines the following terms:
outline-color
[CSS-VALUES-5]
defines the following terms:
attr()
mix()
toggle()
[CSS-VARIABLES-2]
defines the following terms:
custom property
var()
[CSS-WRITING-MODES-4]
defines the following terms:
block axis
inline axis
text-orientation
upright
vertical-lr
vertical-rl
writing mode
writing-mode
[CSS2]
defines the following terms:

border-collapse
border-spacing
line-height
[CSS3-BACKGROUND]
defines the following terms:
background
background positioning area
background-attachment
background-image
background-position
background-size
center
[CSS3-IMAGES]
defines the following terms:

linear-gradient()
[CSSOM]
defines the following terms:
CSS declaration block
CSS rule
location
origin-clean flag
parent CSS rule
parentStyleSheet
stylesheet base URL
[DOM]
defines the following terms:
Document
ShadowRoot
node tree
quirks mode
tree order
[FETCH]
defines the following terms:
RequestDestination
client
credentials mode
destination
fetch
initiator type
mode
origin
processResponseConsumeBody
referrer
request
response
URL
use-URL-credentials flag
[HTML]
defines the following terms:
API base URL
base
find a potential indicated element
origin
relevant settings object
the body element
[INFRA]
defines the following terms:
ASCII case-insensitive
code point
concatenate
for each
identical to
implementation-defined
string
[MEDIAQUERIES-5]
defines the following terms:
continuous media
media query
paged media
[URL]
defines the following terms:
fragment
percent-encode after encoding
URL
URL parser
[WEB-ANIMATIONS-1]
defines the following terms:
discrete
References
Normative References
[CSS-2023]
Chris Lilley; et al.
CSS Snapshot 2023
. URL:
[CSS-ANCHOR-POSITION-1]
Tab Atkins Jr.; Elika Etemad; Ian Kilpatrick.
CSS Anchor Positioning Module Level 1
. URL:
[CSS-BORDERS-4]
Elika Etemad; et al.
CSS Borders and Box Decorations Module Level 4
. URL:
[CSS-BOX-4]
Elika Etemad.
CSS Box Model Module Level 4
. URL:
[CSS-CASCADE-5]
Elika Etemad; Miriam Suzanne; Tab Atkins Jr..
CSS Cascading and Inheritance Level 5
. URL:
[CSS-CASCADE-6]
Elika Etemad; Miriam Suzanne; Tab Atkins Jr..
CSS Cascading and Inheritance Level 6
. URL:
[CSS-COLOR-4]
Tab Atkins Jr.; Chris Lilley; Lea Verou.
CSS Color Module Level 4
. URL:
[CSS-COLOR-5]
Chris Lilley; Una Kravets; Lea Verou.
CSS Color Module Level 5
. URL:
[CSS-CONDITIONAL-3]
Chris Lilley; David Baron; Elika Etemad.
CSS Conditional Rules Module Level 3
. URL:
[CSS-COUNTER-STYLES-3]
Tab Atkins Jr..
CSS Counter Styles Level 3
. URL:
[CSS-DISPLAY-4]
Elika Etemad; Tab Atkins Jr..
CSS Display Module Level 4
. URL:
[CSS-EASING-2]
CSS Easing Functions Level 2
. URL:
[CSS-FONTS-4]
Chris Lilley.
CSS Fonts Module Level 4
. URL:
[CSS-FONTS-5]
Chris Lilley.
CSS Fonts Module Level 5
. URL:
[CSS-GRID-2]
Tab Atkins Jr.; et al.
CSS Grid Layout Module Level 2
. URL:
[CSS-IMAGES-4]
Elika Etemad; Tab Atkins Jr.; Lea Verou.
CSS Images Module Level 4
. URL:
[CSS-INLINE-3]
Elika Etemad.
CSS Inline Layout Module Level 3
. URL:
[CSS-MASKING-1]
Dirk Schulze; Brian Birtles; Tab Atkins Jr..
CSS Masking Module Level 1
. URL:
[CSS-OVERFLOW-3]
Elika Etemad; Florian Rivoal.
CSS Overflow Module Level 3
. URL:
[CSS-PAGE-3]
Elika Etemad.
CSS Paged Media Module Level 3
. URL:
[CSS-POSITION-3]
Elika Etemad; Tab Atkins Jr..
CSS Positioned Layout Module Level 3
. URL:
[CSS-SHADOW-1]
CSS Shadow Module Level 1
. Editor's Draft. URL:
[CSS-SHAPES-1]
Alan Stearns; Rossen Atanassov; Noam Rosenthal.
CSS Shapes Module Level 1
. URL:
[CSS-SIZING-3]
Tab Atkins Jr.; Elika Etemad.
CSS Box Sizing Module Level 3
. URL:
[CSS-SYNTAX-3]
Tab Atkins Jr.; Simon Sapin.
CSS Syntax Module Level 3
. URL:
[CSS-TEXT-4]
Elika Etemad; et al.
CSS Text Module Level 4
. URL:
[CSS-TYPED-OM-1]
Tab Atkins Jr.; François Remy.
CSS Typed OM Level 1
. URL:
[CSS-VALUES-5]
Tab Atkins Jr.; Elika Etemad; Miriam Suzanne.
CSS Values and Units Module Level 5
. URL:
[CSS-VARIABLES-2]
CSS Custom Properties for Cascading Variables Module Level 2
. Editor's Draft. URL:
[CSS-WRITING-MODES-4]
Elika Etemad; Koji Ishii.
CSS Writing Modes Level 4
. URL:
[CSS2]
Bert Bos; et al.
Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification
. URL:
[CSS3-BACKGROUND]
Elika Etemad; Brad Kemper.
CSS Backgrounds and Borders Module Level 3
. URL:
[CSS3-FONTS]
John Daggett; Myles Maxfield; Chris Lilley.
CSS Fonts Module Level 3
. URL:
[CSS3-IMAGES]
Tab Atkins Jr.; Elika Etemad; Lea Verou.
CSS Images Module Level 3
. URL:
[CSSOM]
Daniel Glazman; Emilio Cobos Álvarez.
CSS Object Model (CSSOM)
. URL:
[DOM]
Anne van Kesteren.
DOM Standard
. Living Standard. URL:
[FETCH]
Anne van Kesteren.
Fetch Standard
. Living Standard. URL:
[HTML]
Anne van Kesteren; et al.
HTML Standard
. Living Standard. URL:
[INFRA]
Anne van Kesteren; Domenic Denicola.
Infra Standard
. Living Standard. URL:
[MEDIAQUERIES-5]
Tab Atkins Jr.; et al.
Media Queries Level 5
. URL:
[RFC2119]
S. Bradner.
Key words for use in RFCs to Indicate Requirement Levels
. March 1997. Best Current Practice. URL:
[UNICODE]
The Unicode Standard
. URL:
[URL]
Anne van Kesteren.
URL Standard
. Living Standard. URL:
[WEB-ANIMATIONS-1]
Brian Birtles; et al.
Web Animations
. URL:
Non-Normative References
[CSS-ANIMATIONS-1]
David Baron; et al.
CSS Animations Level 1
. URL:
[CSS-BREAK-3]
Rossen Atanassov; Elika Etemad.
CSS Fragmentation Module Level 3
. URL:
[CSS-CASCADE-3]
Elika Etemad; Tab Atkins Jr..
CSS Cascading and Inheritance Level 3
. URL:
[CSS-COLOR-3]
Tantek Çelik; Chris Lilley; David Baron.
CSS Color Module Level 3
. URL:
[CSS-GRID-1]
Tab Atkins Jr.; et al.
CSS Grid Layout Module Level 1
. URL:
[CSS-OVERFLOW-4]
David Baron; Florian Rivoal; Elika Etemad.
CSS Overflow Module Level 4
. URL:
[CSS-RHYTHM-1]
Elika Etemad; Koji Ishii.
CSS Rhythmic Sizing Module Level 1
. URL:
[CSS-TEXT-3]
Elika Etemad; Koji Ishii; Florian Rivoal.
CSS Text Module Level 3
. URL:
[CSS-TEXT-DECOR-4]
Elika Etemad; Koji Ishii.
CSS Text Decoration Module Level 4
. URL:
[CSS-TRANSFORMS-1]
Simon Fraser; et al.
CSS Transforms Module Level 1
. URL:
[CSS-UI-4]
Tab Atkins Jr.; Florian Rivoal.
CSS Basic User Interface Module Level 4
. URL:
[MEDIAQ]
Tab Atkins Jr.; Florian Rivoal.
Media Queries Level 4
. URL:
[QUIRKS]
Simon Pieters.
Quirks Mode Standard
. Living Standard. URL:
[RFC6694]
S. Moonesamy, Ed..
The "about" URI Scheme
. August 2012. Informational. URL:
Issues Index
Possibly reference
find a potential indicated element
but that is defined specifically for
Document
s,
not
ShadowRoot
s.
CSSOM needs to specify how it rounds,
and it’s probably good for CSS functions to round the same way by default.
What behavior should be used?
[Issue #5689]
MDN
calc()
In all current engines.
Firefox
59+
Safari
12+
Chrome
66+
Opera
Edge
79+
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
clamp
In all current engines.
Firefox
75+
Safari
13.1+
Chrome
79+
Opera
Edge
79+
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
max
In all current engines.
Firefox
75+
Safari
11.1+
Chrome
79+
Opera
Edge
79+
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
min
In all current engines.
Firefox
75+
Safari
11.1+
Chrome
79+
Opera
Edge
79+
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
MDN
url()
In all current engines.
Firefox
1+
Safari
1+
Chrome
1+
Opera
7+
Edge
79+
Edge (Legacy)
12+
IE
8+
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
37+
Samsung Internet
Opera Mobile
url
In all current engines.
Firefox
1+
Safari
1+
Chrome
1+
Opera
3.5+
Edge
79+
Edge (Legacy)
12+
IE
3+
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
MDN
length
In all current engines.
Firefox
53+
Safari
13.1+
Chrome
42+
Opera
Edge
79+
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
56+
Samsung Internet
Opera Mobile
length
In all current engines.
Firefox
1+
Safari
1+
Chrome
1+
Opera
3.5+
Edge
79+
Edge (Legacy)
12+
IE
3+
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
37+
Samsung Internet
Opera Mobile
10.1+
MDN
abs
In only one current engine.
Firefox
None
Safari
15.4+
Chrome
None
Opera
Edge
None
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
sign()
In only one current engine.
Firefox
None
Safari
15.4+
Chrome
None
Opera
Edge
None
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
MDN
acos
In all current engines.
Firefox
108+
Safari
15.4+
Chrome
111+
Opera
Edge
111+
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
asin
In all current engines.
Firefox
108+
Safari
15.4+
Chrome
111+
Opera
Edge
111+
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
atan
In all current engines.
Firefox
108+
Safari
15.4+
Chrome
111+
Opera
Edge
111+
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
atan2
In all current engines.
Firefox
108+
Safari
15.4+
Chrome
111+
Opera
Edge
111+
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
cos
In all current engines.
Firefox
108+
Safari
15.4+
Chrome
111+
Opera
Edge
111+
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
sin
In all current engines.
Firefox
108+
Safari
15.4+
Chrome
111+
Opera
Edge
111+
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
tan
In all current engines.
Firefox
108+
Safari
15.4+
Chrome
111+
Opera
Edge
111+
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
MDN
angle-percentage
In all current engines.
Firefox
3.6+
Safari
4+
Chrome
2+
Opera
Edge
79+
Edge (Legacy)
12+
IE
9+
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
2+
Samsung Internet
Opera Mobile
frequency-percentage
In no current engines.
Firefox
None
Safari
None
Chrome
None
Opera
Edge
None
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
length-percentage
In all current engines.
Firefox
1+
Safari
1+
Chrome
1+
Opera
3.5+
Edge
79+
Edge (Legacy)
12+
IE
3+
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
37+
Samsung Internet
Opera Mobile
10.1+
time-percentage
In all current engines.
Firefox
4+
Safari
3.1+
Chrome
1+
Opera
10.1+
Edge
79+
Edge (Legacy)
12+
IE
9+
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
2+
Samsung Internet
Opera Mobile
10.1+
MDN
angle
In all current engines.
Firefox
3.6+
Safari
4+
Chrome
2+
Opera
Edge
79+
Edge (Legacy)
12+
IE
9+
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
2+
Samsung Internet
Opera Mobile
MDN
calc-constant
In all current engines.
Firefox
108+
Safari
15.4+
Chrome
99+
Opera
Edge
99+
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
MDN
calc
In all current engines.
Firefox
16+
Safari
7+
Chrome
26+
Opera
15+
Edge
79+
Edge (Legacy)
12+
IE
9+
Firefox for Android
iOS Safari
Chrome for Android
28+
Android WebView
37+
Samsung Internet
Opera Mobile
14+
MDN
dimension
In all current engines.
Firefox
1+
Safari
1+
Chrome
1+
Opera
3.5+
Edge
79+
Edge (Legacy)
12+
IE
5+
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
37+
Samsung Internet
Opera Mobile
10.1+
MDN
exp
Firefox
112+
Safari
15.4+
Chrome
None
Opera
Edge
None
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
hypot
Firefox
112+
Safari
15.4+
Chrome
None
Opera
Edge
None
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
log
Firefox
112+
Safari
15.4+
Chrome
None
Opera
Edge
None
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
pow
Firefox
112+
Safari
15.4+
Chrome
None
Opera
Edge
None
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
sqrt
Firefox
112+
Safari
15.4+
Chrome
None
Opera
Edge
None
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
MDN
frequency
In no current engines.
Firefox
None
Safari
None
Chrome
None
Opera
Edge
None
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
MDN
integer
In all current engines.
Firefox
1+
Safari
1+
Chrome
1+
Opera
3.5+
Edge
79+
Edge (Legacy)
12+
IE
3+
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
37+
Samsung Internet
Opera Mobile
10.1+
MDN
mod
Firefox
🔰 109+
Safari
15.4+
Chrome
None
Opera
Edge
None
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
MDN
number
In all current engines.
Firefox
1+
Safari
1+
Chrome
1+
Opera
2+
Edge
79+
Edge (Legacy)
12+
IE
5+
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
10.1+
MDN
percentage
In all current engines.
Firefox
1+
Safari
1+
Chrome
1+
Opera
3.5+
Edge
79+
Edge (Legacy)
12+
IE
3+
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
37+
Samsung Internet
Opera Mobile
10.1+
MDN
position_value
In all current engines.
Firefox
1+
Safari
1+
Chrome
1+
Opera
3.5+
Edge
79+
Edge (Legacy)
12+
IE
4+
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
37+
Samsung Internet
Opera Mobile
MDN
rem
Firefox
🔰 109+
Safari
15.4+
Chrome
None
Opera
Edge
None
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
MDN
resolution
Firefox
8+
Safari
None
Chrome
29+
Opera
9.5+
Edge
79+
Edge (Legacy)
12+
IE
9+
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
10.1+
MDN
round
Firefox
preview+
Safari
15.4+
Chrome
None
Opera
Edge
None
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
MDN
string
In all current engines.
Firefox
1+
Safari
1+
Chrome
1+
Opera
3.5+
Edge
79+
Edge (Legacy)
12+
IE
3+
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
37+
Samsung Internet
Opera Mobile
10.1+
MDN
time
In all current engines.
Firefox
4+
Safari
3.1+
Chrome
1+
Opera
10.1+
Edge
79+
Edge (Legacy)
12+
IE
9+
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
2+
Samsung Internet
Opera Mobile
10.1+
CanIUse
Support:
Android Browser
2.1+
Baidu Browser
13.52+
Blackberry Browser
7+
Chrome
4+
Chrome for Android
147+
Edge
12+
Firefox
3.6+
Firefox for Android
150+
IE
11+
IE Mobile
10+
KaiOS Browser
2.5+
Opera
11.6+
Opera Mini
All
Opera Mobile
12+
QQ Browser
14.9+
Safari
5+
Safari on iOS
6.0+
Samsung Internet
4+
UC Browser for Android
15.5+
Source:
caniuse.com
as of 2026-04-22
CanIUse
Support:
Android Browser
4.4+
Baidu Browser
13.52+
Blackberry Browser
10+
Chrome
27+
Chrome for Android
147+
Edge
12+
Firefox
2+
Firefox for Android
150+
IE (limited)
9+
IE Mobile
10+
KaiOS Browser
2.5+
Opera
15+
Opera Mini
None
Opera Mobile
80+
QQ Browser
14.9+
Safari
7+
Safari on iOS
7.0+
Samsung Internet
4+
UC Browser for Android
15.5+
Source:
caniuse.com
as of 2026-04-22
CanIUse
Support:
Android Browser
4.4+
Baidu Browser
13.52+
Blackberry Browser
10+
Chrome
26+
Chrome for Android
147+
Edge
16+
Firefox
19+
Firefox for Android
150+
IE (limited)
9+
IE Mobile (limited)
10+
KaiOS Browser
2.5+
Opera
15+
Opera Mini
None
Opera Mobile
80+
QQ Browser
14.9+
Safari
6.1+
Safari on iOS
8+
Samsung Internet
4+
UC Browser for Android
15.5+
Source:
caniuse.com
as of 2026-04-22
CanIUse
Support:
Android Browser
147+
Baidu Browser
13.52+
Blackberry Browser
10+
Chrome
26+
Chrome for Android
147+
Edge
12+
Firefox
16+
Firefox for Android
150+
IE (limited)
9+
IE Mobile
10+
KaiOS Browser
2.5+
Opera
15+
Opera Mini
None
Opera Mobile
80+
QQ Browser
14.9+
Safari
6.1+
Safari on iOS
7.0+
Samsung Internet
4+
UC Browser for Android
15.5+
Source:
caniuse.com
as of 2026-04-22