CSS Speech Module Level 1
CSS Speech Module Level 1
Editor’s Draft
22 December 2025
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Editors:
Léonie Watson
Tetralogical
Elika J. Etemad / fantasai
Apple
Former Editors:
Daniel Weck
DAISY Consortium
Claudio Santambrogio
Opera Software
Dave Raggett
W3C / Canon
Suggest an Edit for this Spec:
GitHub Editor
World Wide Web Consortium
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and
permissive document license
rules apply.
Abstract
The Speech module defines aural CSS properties that enable authors to declaratively control the rendering of documents via speech synthesis, and using optional audio cues. Note that this standard was developed in cooperation with the
Voice Browser Activity
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-speech” in the title, like this:
“[css-speech]
…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
The following features are at-risk, and may be dropped during the CR period:
voice-balance
voice-duration
voice-pitch
voice-range
, and
voice-stress
“At-risk” is a W3C Process term-of-art, and does not necessarily imply that the feature is in danger of being dropped or delayed. It means that the WG believes the feature may have difficulty being interoperably implemented in a timely manner, and marking it as such allows the WG to drop the feature if necessary when transitioning to the Proposed Rec stage, without having to publish a new Candidate Rec without the feature first.
1.
Introduction, design goals
This section is non-normative.
The aural presentation of information
is commonly used by people who are
blind, visually-impaired, or otherwise print-disabled.
For instance,
“screen readers” allow users to interact with visual interfaces
that would otherwise be inaccessible to them.
There are also circumstances in which
listening
to content
(as opposed to
reading
is preferred, or sometimes even required,
irrespective of a person’s physical ability to access information.
For instance: playing an e-book whilst driving a vehicle,
learning how to manipulate industrial and medical devices,
interacting with home entertainment systems,
teaching young children how to read.
The CSS properties defined in this Speech module
enable authors to declaratively control the presentation of a document
in the aural dimension.
The aural rendering of a document combines speech synthesis
(also known as “TTS”, the acronym for “Text to Speech”)
and auditory icons
(which are referred-to as “audio cues” in this specification).
The CSS Speech properties provide the ability
to control speech pitch and rate, sound levels, TTS voices, etc.
These stylesheet properties can be used together
with visual properties (mixed media),
or as a complete aural alternative to a visual presentation.
2.
Background information, CSS 2.1
This section is non-normative.
The CSS Speech module is a re-work of the informative
CSS2.1 Aural appendix
within which the
aural
media type was described,
but also deprecated (in favor of the
speech
media type, which has now
also been deprecated).
Although the
[CSS2]
specification reserved the
speech
media type,
it didn’t actually define the corresponding properties.
The Speech module describes the CSS properties that apply to speech output,
and defines a new “box” model specifically for the aural dimension.
Content creators can include CSS properties for user agents with
text to speech synthesis capabilities for any media type - though
generally, they will only make sense for
all
and
screen
These styles are simply ignored by user agents that do not support
the Speech module.
3.
Relationship with SSML
This section is non-normative.
Some of the features in this specification are conceptually similar to
functionality described in the Speech Synthesis Markup Language (SSML) Version 1.1
[SSML]
However, the specificities of the CSS model mean
that compatibility with SSML in terms of syntax and/or semantics
is only partially achievable.
The definition of each property in the Speech module
includes informative statements, wherever necessary,
to clarify their relationship with similar functionality from SSML.
3.1.
Value Definitions
This specification follows the
CSS property definition conventions
from
[CSS2]
using the
value definition syntax
from
[CSS-VALUES-3]
Value types not defined in this specification are defined in CSS Values & Units
[CSS-VALUES-3]
Combination with other CSS modules may expand the definitions of these value types.
In addition to the property-specific values listed in their definitions,
all properties defined in this specification
also accept the
CSS-wide keywords
as their property value.
For readability they have not been repeated explicitly.
4.
Example
This example shows how authors can tell the speech synthesizer to speak HTML headings
with a voice called "paul",
using "moderate" emphasis (which is more than normal)
and how to insert an audio cue (pre-recorded audio clip located at the given URL)
before the start of TTS rendering for each heading.
In a stereo-capable sound system,
paragraphs marked with the CSS class
heidi
are rendered on the left audio channel (and with a female voice, etc.),
whilst the class
peter
corresponds to the right channel (and to a male voice, etc.).
The volume level of text spans marked with the class
special
is lower than normal,
and a prosodic boundary is created
by introducing a strong pause after it is spoken
(note how the
span
inherits the voice-family from its parent paragraph).
h1, h2, h3, h4, h5, h6 {
voice-family: paul;
voice-stress: moderate;
cue-before: url(../audio/ping.wav);
voice-volume: medium 6dB;
p.heidi {
voice-family: female;
voice-balance: left;
voice-pitch: high;
voice-volume: -6dB;
p.peter {
voice-family: male;
voice-balance: right;
voice-rate: fast;
span.special {
voice-volume: soft;
pause-after: strong;

I am Paul, and I speak headings.

Hello, I am Heidi.

Can you hear me ?
I am Peter.

5.
The aural formatting model
The CSS formatting model for aural media is based on
a sequence of sounds and silences that occur within a nested context
similar to the
visual box model
which we name the
aural “box” model
The aural “canvas” consists of a two-channel (stereo) space
and of a temporal dimension,
within which synthetic speech and audio cues coexist.
The selected element is surrounded by
rest
cue
and
pause
properties
(from the innermost to the outermost position).
These can be seen as aural equivalents to
padding
border
and
margin
, respectively.
When used, the
::before
and
::after
pseudo-elements
[CSS2]
get inserted between the element’s contents and the
rest
The following diagram illustrates the equivalence between
properties of the visual and aural box models,
applied to the selected :
6.
Mixing properties
6.1.
The
voice-volume
property
Name:
voice-volume
Value:
silent
[ [ x-soft
soft
medium
loud
x-loud ]
||

Initial:
medium
Applies to:
all elements
Inherited:
yes
Percentages:
N/A
Computed value:
silent
, or a keyword value and optionally also a decibel offset (if not zero)
Canonical order:
per grammar
Animation type:
by computed value type
The
voice-volume
property allows authors to control
the amplitude of the audio waveform generated by the speech synthesizer,
and is also used to adjust the relative volume level of
audio cues
within the
aural box model
of the selected element.
Note:
Although the functionality provided by this property is similar to
the
volume
attribute of the
prosody
element
from the SSML markup language
[SSML]
there are notable discrepancies.
For example, CSS Speech volume keywords and decibels units are not mutually-exclusive,
due to how values are inherited and combined for selected elements.
silent
Specifies that no sound is generated (the text is read "silently").
Note:
This has the same effect as using negative infinity decibels.
Also note that there is a difference between
an element whose
voice-volume
property has a value of
silent
and an element whose
speak
property has the value
none
With the former,
the selected element takes up the same time as if it was spoken,
including any pause before and after the element,
but no sound is generated
(and descendants within the
aural box model
of the selected element
can override the
voice-volume
value, and may therefore generate audio output).
With the latter,
the selected element is not rendered in the aural dimension
and no time is allocated for playback
(descendants within the
aural box model
of the selected element
can override the
speak
value,
and may therefore generate audio output).
x-soft
soft
medium
loud
x-loud
This sequence of keywords corresponds to
monotonically non-decreasing volume levels,
mapped to implementation-dependent values
that meet the listener’s requirements with regards to perceived loudness.
These audio levels are typically provided via a preference mechanism
that allow users to calibrate sound options
according to their auditory environment.
The keyword
x-soft
maps to the user’s
minimum audible
volume level,
x-loud
maps to the user’s
maximum tolerable
volume level,
medium
maps to the user’s
preferred
volume level,
soft
and
loud
map to intermediary values.

This represents a change (positive or negative)
relative to the given keyword value (see enumeration above),
or to the default value for the root element,
or otherwise to the inherited volume level
(which may itself be a combination of a keyword value and decibel offset,
in which case the decibel values are combined additively).
When the inherited volume level is
silent
this
voice-volume
resolves to
silent
too,
regardless of the specified

value.
The

type denotes
dimension
with a "dB" (decibel unit) unit identifier.
Decibels represent
the ratio of the squares of the new signal amplitude
a1
and the current amplitude
a0
as per the following logarithmic equation:
volume(dB) = 20 × log10(
a1
a0
).
Note:
-6.0dB is approximately half the amplitude of the audio signal,
and +6.0dB is approximately twice the amplitude.
Note:
Perceived loudness depends on various factors,
such as the listening environment, user preferences or physical abilities.
The effective volume variation between
x-soft
and
x-loud
represents
the dynamic range (in terms of loudness) of the audio output.
Typically, this range would be compressed in a noisy context,
i.e. the perceived loudness corresponding to
x-soft
would effectively be closer to
x-loud
than it would be in a quiet environment.
There may also be situations where both
x-soft
and
x-loud
would map to low volume levels,
such as in listening environments requiring discretion
(e.g. library, night-reading).
6.2.
The
voice-balance
property
Name:
voice-balance
Value:

left
center
right
leftwards
rightwards
Initial:
center
Applies to:
all elements
Inherited:
yes
Percentages:
N/A
Computed value:
the specified value resolved to a between
-100
and
100
(inclusive)
Canonical order:
per grammar
Animation type:
by computed value type
The
voice-balance
property controls the spatial distribution
of audio output across a lateral sound stage:
one extremity is on the left, the other extremity is on the right hand side,
relative to the listener’s position.
Authors can specify intermediary steps between left hand right extremities,
to represent the audio separation along the resulting left-right axis.
Note:
The functionality provided by this property has no match in the SSML markup language
[SSML]

number
between
-100
and
100
(inclusive).
Values smaller than
-100
are clamped to
-100
Values greater than
100
are clamped to
100
The value
-100
represents the left side,
and the value
100
represents the right side.
The value
represents the center point
whereby there is no discernible audio separation
between left and right sides.
(In a stereo sound system,
this corresponds to equal distribution of audio signals
between left and right speakers).
left
Same as
-100
center
Same as
right
Same as
100
leftwards
Moves the sound to the left
by subtracting 20 from the inherited
voice-balance
value
(and by clamping the resulting number to
-100
).
rightwards
Moves the sound to the right,
by adding 20 to the inherited
voice-balance
value
(and by clamping the resulting number to
100
).
User agents can be connected to different kinds of sound systems,
featuring varying audio mixing capabilities.
The expected behavior for mono, stereo, and surround sound systems
is defined as follows:
When user agents produce audio via a mono-aural sound system
(i.e. single-speaker setup),
the
voice-balance
property has no effect.
When user agents produce audio through a stereo sound system
(e.g. two speakers, or a pair of headphones),
the left-right distribution of audio signals
can precisely match the authored values for the
voice-balance
property.
When user agents are capable of mixing audio signals through more than 2 channels
(e.g. 5-speakers surround sound system, including a dedicated center channel),
the physical distribution of audio signals
resulting from the application of the
voice-balance
property
should be performed so that the listener perceives sound
as if it was coming from a basic stereo layout.
For example, the center channel as well as the left/right speakers
may be used all together
in order to emulate the behavior of the
center
value.
Future revisions of the CSS Speech module may include support for three-dimensional audio,
which would effectively enable authors to specify “azimuth” and “elevation” values.
In the future, content authored using the current specification
may therefore be consumed by user agents which are compliant
with the version of CSS Speech that supports three-dimensional audio.
In order to prepare for this possibility,
the values enabled by the current
voice-balance
property
are designed to remain compatible with “azimuth” angles.
More precisely, the mapping between the current left-right audio axis (lateral sound stage)
and the envisioned 360 degrees plane around the listener’s position
is defined as follows:
The value
maps to zero degrees (
center
).
This is in "front" of the listener, not from "behind".
The value
-100
maps to -40 degrees (
left
).
Negative angles are in the counter-clockwise direction
(assuming the audio stage is seen from the top).
The value
100
maps to 40 degrees (
right
).
Positive angles are in the clockwise direction
(assuming the audio stage is seen from the top).
Intermediary values on the scale from
100
to
100
map to the angles between -40 and 40 degrees
in a numerically linearly-proportional manner.
For example,
-50
maps to -20 degrees.
Note:
Sound systems can be configured by users
in such a way that it would interfere with the left-right audio distribution
specified by document authors.
Typically, the various “surround” modes available in modern sound systems
(including systems based on basic stereo speakers)
tend to greatly alter the perceived spatial arrangement of audio signals.
The illusion of a three-dimensional sound stage
is often achieved using a combination of
phase shifting, digital delay, volume control (channel mixing), and other techniques.
Some users may even configure their system to “downgrade” any rendered sound
to a single mono channel,
in which case the effect of the
voice-balance
property
would obviously not be perceivable at all.
The rendering fidelity of authored content
is therefore dependent on such user customizations,
and the
voice-balance
property merely specifies the desired end-result.
Note:
Many speech synthesizers only generate mono sound,
and therefore do not intrinsically support the
voice-balance
property.
The sound distribution along the left-right axis
consequently occurs at post-synthesis stage
(when the speech-enabled user agent mixes
the various audio sources authored within the document)
7.
Speaking properties
7.1.
The
speak
property
Name:
speak
Value:
auto
never
always
Initial:
auto
Applies to:
all elements
Inherited:
yes
Percentages:
N/A
Computed value:
specified value
Canonical order:
per grammar
Animation type:
discrete
The
speak
property determines whether or not to render text aurally.
Note:
The functionality provided by this property has no match in the SSML markup language
[SSML]
auto
Resolves to a computed value of
never
when
display
is
none
otherwise resolves to a computed value of
auto
The used value of a computed
auto
is equivalent
to
always
if
visibility
is
visible
and to
never
otherwise.
Note:
The
none
value of the
display
property
cannot be overridden by descendants of the selected element,
but the
auto
value of
speak
can, however,
be overridden using either of
never
or
always
never
This value causes an element (including pauses, cues, rests and actual content)
to not be rendered (i.e., the element has no effect in the aural dimension).
Note:
Any of the descendants of the affected element are allowed to override this value,
so descendants can actually take part in the aural rendering
despite using
display: none
at this level.
However, the pauses, cues, and rests of the ancestor element
remain “deactivated” in the aural dimension,
and therefore do not contribute to the
collapsing of pauses
or additive behavior of adjoining rests.
always
The element is rendered aurally
(regardless of its
display
value,
or the
display
or
speak
values of its ancestors).
Note:
Using this value can result in the element being rendered in the aural dimension
even though it would not be rendered on the visual canvas.
7.2.
The
speak-as
property
Name:
speak-as
Value:
normal
spell-out
||
digits
||
[ literal-punctuation
no-punctuation ]
Initial:
normal
Applies to:
all elements
Inherited:
yes
Percentages:
N/A
Computed value:
specified value
Canonical order:
per grammar
Animation type:
discrete
The
speak-as
property determines in what manner text gets rendered aurally,
based upon a predefined list of possibilities.
Note:
The functionality provided by this property is conceptually similar to
the
say-as
element
from the SSML markup language
[SSML]
(whose possible values are described in the
[SSML-SAYAS]
W3C Note).
Although the design goals are similar,
the CSS model is limited to a basic set of pronunciation rules.
normal
Uses language-dependent pronunciation rules for rendering the element’s content.
For example, punctuation is not spoken as-is,
but instead rendered naturally as appropriate pauses.
spell-out
Spells the text one letter at a time (useful for acronyms and abbreviations).
In languages where accented characters are rare,
it is permitted to drop accents in favor of alternative unaccented spellings.
As an example, in English, the word “rôle” can also be written as “role”.
A conforming implementation would thus be able to spell-out “rôle” as “R O L E”.
digits
Speak numbers one digit at a time,
for instance, “twelve” would be spoken as “one two”,
and “31” as “three one”.
Note:
Speech synthesizers are knowledgeable about what a
number
is.
The
speak-as
property enables some level of control on how user agents render numbers,
and may be implemented as a preprocessing step
before passing the text to the actual speech synthesizer.
literal-punctuation
Punctuation such as semicolons, braces, and so on
is named aloud (i.e. spoken literally)
rather than rendered naturally as appropriate pauses.
no-punctuation
Punctuation is not rendered: neither spoken nor rendered as pauses.
8.
Pause properties
8.1.
The
pause-before
and
pause-after
properties
Name:
pause-before
pause-after
Value:

none
x-weak
weak
medium
strong
x-strong
Initial:
none
Applies to:
all elements
Inherited:
no
Percentages:
N/A
Computed value:
specified value
Canonical order:
per grammar
Animation type:
by computed value type
The
pause-before
and
pause-after
properties specify a prosodic boundary
(silence with a specific duration)
that occurs before (or after) the speech synthesis rendition of the element,
or if any
cue-before
(or
cue-after
) is specified,
before (or after) the cue within the
aural box model
Note Although the functionality provided by this property is similar to
the
break
element
from the SSML markup language
[SSML]
the application of
pause
prosodic boundaries within the
aural box model
of CSS Speech
requires special considerations (e.g.
"collapsed" pauses
).

Expresses the pause in absolute time units
(seconds and milliseconds, e.g. "+3s", "250ms").
Only non-negative values are allowed.
none
Equivalent to 0ms (no prosodic break is produced by the speech processor).
x-weak
weak
medium
strong
, and
x-strong
Expresses the pause by the strength of the prosodic break in speech output.
The exact time is implementation-dependent.
The values indicate monotonically non-decreasing (conceptually increasing)
break strength between elements.
Note:
Stronger content boundaries are typically accompanied by pauses.
For example, the breaks between paragraphs are typically much more substantial
than the breaks between words within a sentence.
This example illustrates how the default strengths of prosodic breaks
for specific elements (which are defined by the user agent stylesheet)
can be overridden by authored styles.
p { pause: none } /* pause-before: none; pause-after: none */
8.2.
The
pause
shorthand property
Name:
pause
Value:
<'pause-before'>
<'pause-after'>
Initial:
see individual properties
Applies to:
all elements
Inherited:
no
Percentages:
N/A
Computed value:
see individual properties
Animation type:
see individual properties
Canonical order:
per grammar
The
pause
property is a shorthand property for
pause-before
and
pause-after
If two values are given, the first value is
pause-before
and the second is
pause-after
If only one value is given, it applies to both properties.
Examples of property values:
h1 { pause: 20ms; } /* pause-before: 20ms; pause-after: 20ms */
h2 { pause: 30ms 40ms; } /* pause-before: 30ms; pause-after: 40ms */
h3 { pause-after: 10ms; } /* pause-before:
unspecified
; pause-after: 10ms */
8.3.
Collapsing pauses
The pause defines the minimum distance of the aural "box"
to the aural "boxes" before and after it.
Adjoining pauses are merged by selecting the strongest named break
and the longest absolute time interval.
For example, "strong" is selected when merging "strong" and "weak",
"1s" is selected when merging "1s" and "250ms",
and "strong" and "250ms" take effect additively when merging "strong" and "250ms".
The following pauses are adjoining:
The
pause-after
of an aural "box" and the
pause-after
of its last child,
provided the former has no
rest-after
and no
cue-after
The
pause-before
of an aural "box" and the
pause-before
of its first child,
provided the former has no
rest-before
and no
cue-before
The
pause-after
of an aural "box" and the
pause-before
of its next sibling.
The
pause-before
and
pause-after
of an aural "box",
if the "box" has a
voice-duration
of "0ms"
and no
rest-before
or
rest-after
and no
cue-before
or
cue-after
or if the "box" has no rendered content at all (see
speak
).
A collapsed pause is considered adjoining to another pause
if any of its component pauses is adjoining to that pause.
Note:
pause
has been moved from between the element’s contents and any
cue
to outside the
cue
This is not backwards compatible with the informative CSS2.1 Aural appendix
[CSS2]
9.
Rest properties
9.1.
The
rest-before
and
rest-after
properties
Name:
rest-before
rest-after
Value:

none
x-weak
weak
medium
strong
x-strong
Initial:
none
Applies to:
all elements
Inherited:
no
Percentages:
N/A
Computed value:
specified value
Canonical order:
per grammar
Animation type:
by computed value type
The
rest-before
and
rest-after
properties specify a prosodic boundary
(silence with a specific duration)
that occurs before (or after) the speech synthesis rendition of an element within the
aural box model
Note:
Although the functionality provided by this property is similar to
the
break
element
from the SSML markup language
[SSML]
the application of
rest
prosodic boundaries within the
aural box model
of CSS Speech
requires special considerations (e.g. interspersed audio cues, additive adjacent rests).

Expresses the rest in absolute time units (seconds and milliseconds, e.g. "+3s", "250ms").
Only non-negative values are allowed.
none
Equivalent to 0ms.
(No prosodic break is produced by the speech processor.)
x-weak
weak
medium
strong
, and
x-strong
Expresses the rest by the strength of the prosodic break in speech output.
The exact time is implementation-dependent.
The values indicate monotonically non-decreasing (conceptually increasing)
break strength between elements.
As opposed to
pause properties
the rest is inserted between the element’s content
and any
cue-before
or
cue-after
content.
Adjoining rests are treated additively, and do not collapse.
9.2.
The
rest
shorthand property
Name:
rest
Value:
<'rest-before'>
<'rest-after'>
Initial:
see individual properties
Applies to:
all elements
Inherited:
no
Percentages:
N/A
Computed value:
see individual properties
Animation type:
see individual properties
Canonical order:
per grammar
The
rest
property is a shorthand for
rest-before
and
rest-after
If two values are given, the first value is
rest-before
and the second is
rest-after
If only one value is given, it applies to both properties.
10.
Cue properties
10.1.
The
cue-before
and
cue-after
properties
Name:
cue-before
cue-after
Value:

none
Initial:
none
Applies to:
all elements
Inherited:
no
Percentages:
N/A
Computed value:
specified value
Canonical order:
per grammar
Animation type:
by computed value type
The
cue-before
and
cue-after
properties specify auditory icons
(i.e. pre-recorded / pre-generated sound clips)
to be played before (or after) the element within the
aural box model
Note:
Although the functionality provided by this property may appear related to
the
audio
element
from the SSML markup language
[SSML]
there are in fact major discrepancies.
For example, the
aural box model
means that
audio cues are associated to the element’s volume level;
and CSS Speech’s auditory icons provide limited functionality
compared to SSML’s
audio
element.

The URI designates an auditory icon resource.
When a user agent is not able to render the specified auditory icon
(e.g. missing file resource, or unsupported audio codec),
it is recommended to produce an alternative cue, such as a bell sound.
none
Specifies that no auditory icon is used.

Represents a change (positive or negative) relative to
the computed value of the
voice-volume
property
within the
aural box model
of the selected element.
(As a result, the volume level of an audio cue changes
when the
voice-volume
property changes).
When omitted, the implied value computes to 0dB.
When the computed value of the
voice-volume
property is
silent
the audio cue is also set to
silent
(regardless of this specified

value).
Otherwise (when not
silent
),
voice-volume
values are always specified relatively
to the volume level keywords (see the definition of
voice-volume
),
which map to a user-calibrated scale of "preferred" loudness settings.
If the inherited
voice-volume
value already contains a decibel offset,
the dB offset specific to the audio cue is combined additively.
Note:
There is a difference between an audio cue
whose volume is set to
silent
and one whose value is
none
In the former case, the audio cue takes up the same time as if it had been played,
but no sound is generated.
In the latter case, the there is no manifestation of the audio cue at all
(i.e. no time is allocated for the cue in the aural dimension).
Examples of property values:
cue-before: url(/audio/bell.aiff) -3dB;
cue-after: url(dong.wav);

h1
cue-before: url(../clips-1/pop.au) +6dB;
cue-after: url(../clips-2/pop.au) 6dB;

div.caution { cue-before: url(./audio/caution.wav) +8dB; }
10.2.
Relation between audio cues and speech synthesis volume levels
This section is non-normative.
The volume levels of audio cues and of speech synthesis
within the
aural box model
of a selected element are related.
For example, the desired effect of an audio cue
whose volume level is set at +0dB (as specified by the

value)
is that its perceived loudness during playback
is close to that of the speech synthesis rendition of the selected element,
as dictated by the computed value of the
voice-volume
property.
Note that a
silent
computed value for the
voice-volume
property
results in audio cues being "forcefully" silenced as well
(i.e. regardless of the specified audio cue

value)
The volume keywords of the
voice-volume
property
are user-calibrated to match requirements not known at authoring time
(e.g. auditory environment, personal preferences).
Therefore, in order to achieve this approximate loudness alignment of audio cues and speech synthesis,
authors should ensure that the volume level of audio cues
(on average, as there may be discrete variations of perceived loudness
due to changes in the audio stream, such as intonation, stress, etc.)
matches the output of a speech synthesis rendition based on the
voice-family
intended for use,
given "typical" listening conditions
(i.e. default system volume levels, centered equalization across the frequency spectrum).
As speech processors are capable of directly controlling
the waveform amplitude of generated text-to-speech audio,
and because user agents are able to adjust the volume output of audio cues
(i.e. amplify or attenuate audio signals based on the intrinsic waveform amplitude of digitized sound clips),
this sets a baseline that enables implementations to manage the loudness
of both TTS and cue audio streams within the aural box model,
relative to user-calibrated volume levels
(see the keywords defined in the
voice-volume
property).
Due to the complex relationship between perceived audio characteristics (e.g. loudness)
and the processing applied to the digitized audio signal (e.g. signal compression),
we refer to a simple scenario whereby the attenuation is indicated in decibels,
typically ranging from 0dB (i.e. maximum audio input, near clipping threshold)
to -60dB (i.e. total silence).
Given this context, a "standard" audio clip would oscillate between these values,
the loudest peak levels would be close to -3dB (to avoid distortion),
and the relevant audible passages would have average (RMS) volume levels
as high as possible (i.e. not too quiet, to avoid background noise during amplification).
This would roughly provide an audio experience that could be
seamlessly combined with text-to-speech output
(i.e. there would be no discernible difference in volume levels
when switching from pre-recorded audio to speech synthesis).
Although there exists no industry-wide standard to support such convention,
different TTS engines tend to generate comparably-loud audio signals
when no gain or attenuation is specified.
For voice and soft music, -15dB RMS seems to be pretty standard.
10.3.
The
cue
shorthand property
Name:
cue
Value:
<'cue-before'>
<'cue-after'>
Initial:
see individual properties
Applies to:
all elements
Inherited:
no
Percentages:
N/A
Computed value:
see individual properties
Animation type:
see individual properties
Canonical order:
per grammar
The
cue
property is a shorthand for
cue-before
and
cue-after
If two values are given the first value is
cue-before
and the second is
cue-after
If only one value is given, it applies to both properties.
Example of shorthand notation:
h1
cue-before: url(pop.au);
cue-after: url(pop.au);
/* ...is equivalent to: */
h1
cue: url(pop.au);
11.
Voice characteristic properties
11.1.
The
voice-family
property
Name:
voice-family
Value:

preserve
Initial:
implementation-dependent
Applies to:
all elements
Inherited:
yes
Percentages:
N/A
Computed value:
specified value
Canonical order:
per grammar
Animation type:
discrete
The
voice-family
property specifies a prioritized list of component values
that are separated by commas to indicate that they are alternatives.
(This is analogous to
font-family
in visual style sheets.)
Each component value potentially designates a speech synthesis voice instance,
by specifying match criteria.
See the
voice selection
section on this topic.

Note:
Although the functionality provided by this property is similar to
the
voice
element
from the SSML markup language
[SSML]
CSS Speech does not provide an equivalent to SSML’s sophisticated voice language selection.
This technical limitation may be alleviated in a future revision of the Speech module.

Values are specific voice instances (e.g., Mike, comedian, mary, carlos2, "valley girl").
Like
font-family
names, voice names must either be given quoted as
strings
or unquoted as a sequence of one or more
CSS identifiers
Note:
As a result, most punctuation characters, or digits at the start of each token,
must be escaped in unquoted voice names.
If a sequence of identifiers is given as a voice name,
the computed value is the name converted to a string
by joining all the identifiers in the sequence by single spaces.
Voice names that happen to be the same as the gender keywords
male
female
and
neutral
or that happen to match the
CSS-wide keywords
or
preserve
must be quoted to disambiguate with these keywords.
The keyword
default
is reserved for future use and must also be quoted when used as voice names.
Note:
In
[SSML]
, voice names are space-separated and cannot contain whitespace characters.
It is recommended to quote voice names that contain
white space, digits, or punctuation characters other than hyphens—even if these voice names are valid in unquoted form—in order to improve code clarity.
For example:
voice-family: "john doe", "Henry the-8th";

Possible values are
child
young
and
old
indicating the preferred age category to match during voice selection.
Note:
A recommended mapping with
[SSML]
ages is:
child
= 6 y/o,
young
= 24 y/o,
old
= 75 y/o.
More flexible age ranges may be used by the processor-dependent voice-matching algorithm.

One of the keywords
male
female
, or
neutral
specifying a male, female, or neutral voice, respectively.
Note:
The interpretation of the relationship between a person’s age or gender,
and a recognizable type of voice,
cannot realistically be defined in a universal manner as it effectively depends on numerous criteria
(cultural, linguistic, biological, etc.).
The functionality provided by this specification therefore represent
a simplified model that can be reasonably applied to a broad variety of speech contexts,
albeit at the cost of a certain degree of approximation.
Future versions of this specification may refine the level of precision of the voice-matching algorithm,
as speech processor implementations become more standardized.

An integer indicating the preferred variant (e.g. "the second male child voice").
Only positive integers (i.e. excluding zero) are allowed.
The value
refers to the first of all matching voices.
preserve
Indicates that the
voice-family
value gets inherited and used
regardless of any potential language change within the content markup
(see the section below about voice selection and language handling).
This value behaves as
inherit
when applied to the root element.
Note: Descendants of the element automatically inherit the
preserve
value,
unless it is explicitly overridden by other
voice-family
values (e.g. name, gender, age).
Examples of invalid declarations:
voice-family: john/doe; /* forward slash character should be escaped */
voice-family: john "doe"; /* identifier sequence cannot contain strings */
voice-family: john!; /* exclamation mark should be escaped */
voice-family: john@doe; /* "at" character should be escaped */
voice-family: #john; /* identifier cannot start with hash character */
voice-family: john 1st; /* identifier cannot start with digit */
11.1.1.
Voice selection, content language
The
voice-family
property is used to guide the selection of the speech synthesis voice instance.
As part of this selection process, speech-capable user agents must also take into account
the language of the selected element within the markup content.
The "name", "gender", "age", and preferred "variant" (index)
are voice selection hints that get carried down the content hierarchy
as the
voice-family
property value gets inherited by descendant elements.
At any point within the content structure,
the language takes precedence (i.e. has a higher priority)
over the specified CSS voice characteristics.
The following list outlines the voice selection algorithm
(note that the definition of "language" is loose here,
in order to cater for dialectic variations):
If only a single voice instance is available for the language of the selected content,
then this voice must be used, regardless of the specified CSS voice characteristics.
If several voice instances are available for the language of the selected content,
then the chosen voice is the one that most closely matches
the specified name, or gender, age, and preferred voice variant.
The actual definition of "best match" is processor-dependent.
For example, in a system that only has male and female adult voices available,
a reasonable match for "voice-family: young male"
may well be a higher-pitched female voice,
as this tone of voice would be closer to that of a young boy.
If no voice instance matches the characteristics
provided by any of the
voice-family
component values,
the first available voice instance
(amongst those suitable for the language of the selected content)
must be used.
If no voice is available for the language of the selected content,
it is recommended that user agents let the user know
about the lack of appropriate TTS voice.
The speech synthesizer voice must be re-evaluated
(i.e. the selection process must take place once again)
whenever any of the CSS voice characteristics change within the content flow.
The voice must also be re-calculated whenever the content language changes,
unless the
preserve
keyword is used
(this may be useful in cases where embedded foreign language text
can be spoken using a voice not designed for this language,
as demonstrated by the example below).
Note:
Dynamically computing a voice may lead to unexpected lag,
so user agents should try to resolve concrete voice instances in the document tree
before the playback starts.
Examples of property values:
h1 { voice-family: announcer, old male; }
p.romeo { voice-family: romeo, young male; }
p.juliet { voice-family: juliet, young female; }
p.mercutio { voice-family: young male; }
p.tybalt { voice-family: young male; }
p.nurse { voice-family: amelie; }

...

The French text below will be spoken with an English voice:
Bonjour monsieur !

The English text below will be spoken with a voice different
than that corresponding to the class "romeo"
(which is inherited from the "p" parent element):
Hello sir!

11.2.
The
voice-rate
property
Name:
voice-rate
Value:
[ normal
x-slow
slow
medium
fast
x-fast ]
||

Initial:
normal
Applies to:
all elements
Inherited:
yes
Percentages:
refer to default value
Computed value:
a keyword value, and optionally also a percentage relative to the keyword (if not 100%)
Canonical order:
per grammar
Animation type:
by computed value type
The
voice-rate
property manipulates the rate of generated synthetic speech
in terms of words per minute.
Note:
Although the functionality provided by this property is similar to
the
rate
attribute
of the
prosody
element
from the SSML markup language
[SSML]
there are notable discrepancies.
For example, CSS Speech rate keywords and percentage modifiers are not mutually-exclusive,
due to how values are inherited and combined for selected elements.
normal
Represents the default rate produced by the speech synthesizer for the currently active voice.
This is processor-specific and depends on the language and dialect,
and on the "personality" of the voice.
x-slow
slow
medium
fast
and
x-fast
A sequence of monotonically non-decreasing speaking rates
that are implementation- and voice-specific.
For example, typical values for the English language are
(in words per minute) x-slow = 80, slow = 120, medium = between 180 and 200, fast = 500.

Only non-negative
percentage
values are allowed.
This represents a change relative to the given keyword value (see enumeration above),
or to the default value for the root element,
or otherwise to the inherited speaking rate
(which may itself be a combination of a keyword value and of a percentage,
in which case percentages are combined multiplicatively).
For example, 50% means that the speaking rate gets multiplied by 0.5 (half the value).
Percentages above 100% result in faster speaking rates (relative to the base keyword),
whereas percentages below 100% result in slower speaking rates.
Examples of inherited values:

...

body { voice-rate: inherit; } /* the initial value is 'normal'
(the actual speaking rate value
depends on the active voice) */

e1 { voice-rate: +50%; } /* the computed value is
['normal' and 50%], which will resolve
to the rate corresponding to 'normal'
multiplied by 0.5 (half the speaking rate) */

e2 { voice-rate: fast 120%; } /* the computed value is
['fast' and 120%], which will resolve
to the rate corresponding to 'fast'
multiplied by 1.2 */

e3 { voice-rate: normal; /* "resets" the speaking rate to the intrinsic voice value,
the computed value is 'normal' (see comment below for actual value) */
voice-family: "another-voice"; } /* because the voice is different,
the calculated speaking rate may vary
compared to "body" (even though the computed
'voice-rate' value is the same) */
11.3.
The
voice-pitch
property
Name:
voice-pitch
Value:

&&
absolute
[ [ x-low
low
medium
high
x-high ]
||

] ]
Initial:
medium
Applies to:
all elements
Inherited:
yes
Percentages:
refer to inherited value
Computed value:
one of the predefined pitch keywords if only the keyword is specified by itself,
otherwise an absolute frequency calculated by converting the keyword value (if any) to a
fixed frequency based on the current voice-family and by applying the specified relative
offset (if any)
Canonical order:
per grammar
Animation type:
by computed value type
The
voice-pitch
property specifies the "baseline" pitch of the generated speech output,
which depends on the used
voice-family
instance,
and varies across speech synthesis processors
(it approximately corresponds to the average pitch of the output).
For example, the common pitch for a male voice is around 120Hz,
whereas it is around 210Hz for a female voice.
Note:
Although the functionality provided by this property is similar to
the
pitch
attribute of the
prosody
element
from the SSML markup language
[SSML]
there are notable discrepancies.
For example, CSS Speech pitch keywords and relative changes (frequency, semitone or percentage)
are not mutually-exclusive, due to how values are inherited and combined for selected elements.

A value in frequency units (Hertz or kiloHertz, e.g.
100Hz
+2kHz
).
Values are restricted to positive numbers when the
absolute
keyword is specified.
Otherwise (when the
absolute
keyword is not specified),
a negative value represents a decrement, and a positive value represents an increment,
relative to the inherited value.
For example,
2kHz
is a positive offset (strictly equivalent to
+2kHz
),
and
+2kHz absolute
is an absolute frequency (strictly equivalent to
2kHz absolute
).
absolute
If specified, this keyword indicates that the specified frequency
represents an absolute value.
If a negative frequency is specified, the computed frequency will be zero.

Specifies a relative change (decrement or increment) to the inherited value.
The syntax of

allowed values is
dimension
with the unit identifier
st
(semitones).
semitone
interval corresponds to the step between each note
on an equal temperament chromatic scale.
semitone
can therefore be quantified as the difference between
two consecutive pitch frequencies on such scale.
The ratio between two consecutive frequencies separated by exactly one
semitone
is the twelfth root of two (approximately 11011/10393, which equals exactly 1.0594631).
As a result, the value in Hertz corresponding to a semitone offset
is relative to the initial frequency the offset is applied to.
(In other words, a
semitone
doesn’t correspond to a fixed numerical value in Hertz.)

Positive and negative
percentage
values are allowed,
to represent an increment or decrement (respectively) relative to the inherited value.
Computed values are calculated by adding (or subtracting)
the specified fraction of the inherited value,
to (from) the inherited value.
For example, 50% (which is equivalent to +50%) with a inherited value of 200Hz
results in
200 + (200*0.5)
= 300Hz.
Conversely, -50% results in
200-(200*0.5)
= 100Hz.
x-low
low
medium
high
x-high
A sequence of monotonically non-decreasing pitch levels
that are implementation and voice specific.
When the computed value for a given element is only a keyword
(i.e. no relative offset is specified),
then the corresponding absolute frequency will be re-evaluated on a voice change.
Conversely, the application of a relative offset requires the calculation of the resulting frequency
based on the current voice at the point at which the relative offset is specified,
so the computed frequency will inherit absolutely
regardless of any voice change further down the style cascade.
Authors should therefore only use keyword values
in cases where they wish that voice changes trigger
the re-evaluation of the conversion from a keyword
to a concrete, voice-dependent frequency.
Computed absolute frequencies that are negative are clamped to zero Hertz.
Speech-capable user agents are likely to support a specific range of values
rather than the full range of possible calculated numerical values for frequencies.
The actual values in user agents may therefore be clamped to implementation-dependent minimum and maximum boundaries.
For example, although the 0Hz frequency can be legitimately calculated,
it may be clamped to a more meaningful value in the context of the speech synthesizer.
Examples of property values:
h1 { voice-pitch: 250Hz; } /* positive offset relative to the inherited absolute frequency */
h1 { voice-pitch: +250Hz; } /* identical to the line above */
h2 { voice-pitch: +30Hz absolute; } /* not an increment */
h2 { voice-pitch: absolute 30Hz; } /* identical to the line above */
h3 { voice-pitch: -20Hz; } /* negative offset (decrement) relative to the inherited absolute frequency */
h4 { voice-pitch: -20Hz absolute; } /* illegal syntax => value ignored ("absolute" keyword not allowed with negative frequency) */
h5 { voice-pitch: -3.5st; } /* semitones, negative offset */
h6 { voice-pitch: 25%; } /* this means "add a quarter of the inherited value, to the inherited value" */
h6 { voice-pitch: +25%; } /* identical to the line above */
11.4.
The
voice-range
property
Name:
voice-range
Value:

&&
absolute
[ [ x-low
low
medium
high
x-high ]
||

] ]
Initial:
medium
Applies to:
all elements
Inherited:
yes
Percentages:
refer to inherited value
Computed value:
one of the predefined pitch keywords if only the keyword is specified by itself,
otherwise an absolute frequency calculated by converting the keyword value (if any) to a
fixed frequency based on the current voice-family and by applying the specified relative
offset (if any)
Canonical order:
per grammar
Animation type:
by computed value type
The
voice-range
property specifies the variability in the "baseline" pitch,
i.e. how much the fundamental frequency may deviate from the average pitch of the speech output.
The dynamic pitch range of the generated speech generally increases for a highly animated voice,
for example when variations in inflection are used to convey meaning and emphasis in speech.
Typically, a low range produces a flat, monotonic voice,
whereas a high range produces an animated voice.
Note:
Although the functionality provided by this property is similar to
the
range
attribute of the
prosody
element
from the SSML markup language
[SSML]
there are notable discrepancies.
For example, CSS Speech pitch range keywords and relative changes (frequency, semitone or percentage)
are not mutually-exclusive, due to how values are inherited and combined for selected elements.

A value in frequency units (Hertz or kiloHertz, e.g.
100Hz
+2kHz
).
Values are restricted to positive numbers when the
absolute
keyword is specified.
Otherwise (when the
absolute
keyword is not specified),
a negative value represents a decrement, and a positive value represents an increment,
relative to the inherited value.
For example,
2kHz
is a positive offset (strictly equivalent to
+2kHz
),
and
+2kHz absolute
is an absolute frequency (strictly equivalent to
2kHz absolute
).
absolute
If specified, this keyword indicates that the specified frequency
represents an absolute value.
If a negative frequency is specified, the computed frequency will be zero.

Specifies a relative change (decrement or increment) to the inherited value
as a
semitone

Positive and negative
percentage
values represent
an increment or decrement (respectively) relative to the inherited value.
Computed values are calculated by adding (or subtracting)
the specified fraction of the inherited value,
to (from) the inherited value.
For example, 50% (which is equivalent to +50%) with a inherited value of 200Hz
results in
200 + (200*0.5)
= 300Hz.
Conversely, -50% results in
200-(200*0.5)
= 100Hz.
x-low
low
medium
high
x-high
A sequence of monotonically non-decreasing pitch levels
that are implementation and voice specific.
When the computed value for a given element is only a keyword
(i.e. no relative offset is specified),
then the corresponding absolute frequency will be re-evaluated on a voice change.
Conversely, the application of a relative offset requires the calculation of the resulting frequency
based on the current voice at the point at which the relative offset is specified,
so the computed frequency will inherit absolutely
regardless of any voice change further down the style cascade.
Authors should therefore only use keyword values
in cases where they wish that voice changes trigger
the re-evaluation of the conversion from a keyword
to a concrete, voice-dependent frequency.
Computed absolute frequencies that are negative are clamped to zero Hertz.
Speech-capable user agents are likely to support a specific range of values
rather than the full range of possible calculated numerical values for frequencies.
The actual values in user agents may therefore be clamped
to implementation-dependent minimum and maximum boundaries.
For example: although the 0Hz frequency can be legitimately calculated,
it may be clamped to a more meaningful value in the context of the speech synthesizer.
Examples of inherited values:

...

body { voice-range: inherit; } /* the initial value is 'medium'
(the actual frequency value
depends on the current voice) */

e1 { voice-range: +25%; } /* the computed value is
['medium' + 25%] which resolves
to the frequency corresponding to 'medium'
plus 0.25 times the frequency
corresponding to 'medium' */

e2 { voice-range: +10Hz; } /* the computed value is
[FREQ + 10Hz] where "FREQ" is the absolute frequency
calculated in the "e1" rule above. */

e3 { voice-range: inherit; /* this could be omitted,
but we explicitly specify it for clarity purposes */

voice-family: "another-voice"; } /* this voice change would have resulted in
the re-evaluation of the initial 'medium' keyword
inherited by the "body" element
(i.e. conversion from a voice-dependent keyword value
to a concrete, absolute frequency),
but because relative offsets were applied down the style
cascade, the inherited value is actually the frequency
calculated at the "e2" rule above. */

e4 { voice-range: 200Hz absolute; } /* override with an absolute frequency
which doesn't depend on the current voice */

e5 { voice-range: 2st; } /* the computed value is an absolute frequency,
which is the result of the
calculation: 200Hz + two semitones
(reminder: the actual frequency corresponding to a semitone
depends on the base value to which it applies) */

e6 { voice-range: inherit; /* this could be omitted,
but we explicitly specify it for clarity purposes */

voice-family: "yet-another-voice"; } /* despite the voice change,
the computed value is the same as
for "e5" (i.e. an absolute frequency value,
independent from the current voice) */
11.5.
The
voice-stress
property
Name:
voice-stress
Value:
normal
strong
moderate
none
reduced
Initial:
normal
Applies to:
all elements
Inherited:
yes
Percentages:
N/A
Computed value:
specified value
Canonical order:
per grammar
Animation type:
discrete
The
voice-stress
property manipulates the strength of emphasis,
which is normally applied using a combination of
pitch change, timing changes, loudness and other acoustic differences.
The precise meaning of the values therefore depend on the language being spoken.
Note:
The functionality provided by this property is similar to
the
emphasis
element
from the SSML markup language
[SSML]
normal
Represents the default emphasis produced by the speech synthesizer.
none
Prevents the synthesizer from emphasizing text it would normally emphasize.
moderate
and
strong
These values are monotonically non-decreasing in strength.
Their application results in more emphasis
than what the speech synthesizer would normally produce
(i.e. more than the value corresponding to
normal
).
reduced
Effectively the opposite of emphasizing a word.
Examples of property values, with HTML sample:
.default-emphasis { voice-stress: normal; }
.lowered-emphasis { voice-stress: reduced; }
.removed-emphasis { voice-stress: none; }
.normal-emphasis { voice-stress: moderate; }
.huge-emphasis { voice-stress: strong; }

...

This is a big car.

This is a big car.

This car is massive!

This is a big car!

This is a big car!!!

12.
Voice duration property
12.1.
The
voice-duration
property
Name:
voice-duration
Value:
auto

Initial:
auto
Applies to:
all elements
Inherited:
no
Percentages:
N/A
Computed value:
specified value
Canonical order:
per grammar
Animation type:
by computed value type
The
voice-duration
property specifies how long it should take to render
the selected element’s content
(not including
audio cues
pauses
and
rests
).
Unless the value
auto
is specified,
this property takes precedence over the
voice-rate
property,
and should be used to determine a suitable speaking rate for the voice.
An element for which the
voice-duration
property value is not
auto
can have descendants for which the
voice-duration
and
voice-rate
properties are specified,
but these must be ignored.
In other words, when a

is specified for the
voice-duration
of a selected element,
it applies to the entire element subtree (children cannot override the property).
Note:
The functionality provided by this property is similar to the
the
duration
attribute
of the
prosody
element
from the SSML markup language
[SSML]
auto
Resolves to a used value corresponding to the duration of the speech synthesis
when using the inherited
voice-rate

Specifies a value in absolute time units
(seconds and milliseconds, e.g. "+3s", "250ms").
Only non-negative values are allowed.
13.
List items and counters styles
The
list-style-type
property of
[CSS2]
specifies three types of list item markers:
glyphs, numbering systems, and alphabetic systems.
The values allowed for this property are also used for the counter() function of the
content
property.
The CSS Speech module defines how to render these styles in the aural dimension,
using speech synthesis.
The
list-style-image
property of
[CSS2]
is ignored,
and instead the
list-style-type
is used.
Note:
The speech rendering of new features from the CSS Lists and Counters Module Level 3
[CSS3LIST]
is not covered in this level of CSS Speech, but may be defined in a future specification.
disc
circle
square
For these list item styles,
the user agent defines (possibly based on user preferences)
what equivalent phrase is spoken or what audio cue is played.
List items with graphical bullets are therefore announced appropriately
in an implementation-dependent manner.
decimal
decimal-leading-zero
lower-roman
upper-roman
georgian
armenian
For these list item styles,
corresponding numbers are spoken as-is by the speech synthesizer,
and may be complemented with additional audio cues
or speech phrases in the document’s language
(i.e. with the same TTS voice used to speak the list item content)
in order to indicate the presence of list items.
For example, when using the English language,
the list item counter could be prefixed with the word "Item",
which would result in list items being announced with "Item one", "Item two", etc.
lower-latin
lower-alpha
upper-latin
upper-alpha
lower-greek
These list item styles are spelled out letter-by-letter by the speech synthesizer,
in the document language (i.e. with the same TTS voice used to speak the list item content).
For example,
lower-greek
in English would be read out as "alpha", "beta", "gamma", etc.
Similarly,
upper-latin
in French would be read out as /a/, /be/, /se/, etc. (phonetic notation)
Note:
It is common for user agents such as screen readers to announce the nesting depth of list items,
or more generally, to indicate additional structural information pertaining to complex hierarchical content.
The verbosity of these additional audio cues and/or speech output
can usually be controlled by users, and contribute to increasing usability.
These navigation aids are implementation-dependent,
but it is recommended that user agents supporting the CSS Speech module ensure
that these additional audio cues and speech output don’t generate redundancies
or create inconsistencies
(for example: duplicated or different list item numbering scheme).
14.
Inserted and replaced content
This section is non-normative.
Sometimes, authors will want to specify a mapping from the source text into another string
prior to the application of the regular pronunciation rules.
This may be used for uncommon abbreviations
or acronyms which are unlikely to be recognized by the synthesizer.
The
content
property can be used to replace one string by another.
The functionality provided by this property is similar to
the
alias
attribute
of the
sub
element
from the SSML markup language
[SSML]
In this example, the abbreviation is rendered
using the content of the title attribute instead of the element’s content.
/* This replaces the content of the selected element
by the string "World Wide Web Consortium". */
abbr { content: attr(title); }
...

W3C
In a similar way, text strings in a document can be replaced by a previously recorded version.
In this example—assuming the format is supported, the file is available, and the UA is configured to do so—a recording of Sir John Gielgud’s declamation of the famous monologue is played.
Otherwise the UA falls back to render the text using synthesized speech.
.hamlet { content: url(./audio/gielgud.wav); }
...

To be, or not to be: that is the question:

Furthermore, authors (or users via a user stylesheet) may add some information
to ease the understanding of structures during non-visual interaction with the document.
They can do so by using the
::before
and
::after
pseudo-elements.
Note that different stylesheets can be used to define the level of verbosity
for additional information spoken by screen readers.
This example inserts the string "Start list: " before a list
and the string "List item: " before the content of each list item.
Likewise, the string "List end: " gets inserted after the list
to inform the user that the list speech output is over.
ul::before { content: "Start list: "; }
ul::after { content: "List end. "; }
li::before { content: "List item: "; }
Detailed information can be found in the CSS3 Generated and Replaced Content module
[CSS3GENCON]
15.
Pronunciation, phonemes
This section is non-normative.
CSS does not specify how to define the pronunciation (expressed using a well-defined phonetic alphabet)
of a particular piece of text within the markup document.
A "phonemes" property was described in earlier drafts of this specification,
but objections were raised due to breaking the principle of separation
between content and presentation.
(The "phonemes" authored within aural CSS stylesheets would have needed
to be updated each time text changed within the markup document.)
The "phonemes" functionality is therefore considered out-of-scope in CSS (the presentation layer)
and should be addressed in the markup / content layer.
The
"pronunciation"
rel
value allows importing pronunciation lexicons in HTML documents
using the
link
element (similar to how CSS stylesheets can be included).
The W3C PLS (Pronunciation Lexicon Specification)
[PRONUNCIATION-LEXICON]
is one format that can be used to describe such a lexicon.
Additionally, an attribute-based mechanism can be used within the markup
to author text-pronunciation associations
At the time of writing, such mechanism isn’t formally defined in the W3C HTML standard(s).
However, the
EPUB 3.0 specification
allows
(x)HTML5 documents to contain attributes derived from the
[SSML]
specification,
that describe how to pronounce text based on a particular phonetic alphabet.
Glossary
The following terms and abbreviations are used in this module.
UA
user agent
A program that reads and/or writes CSS style sheets on behalf of a user in either or both
of these categories: programs whose purpose is to render
documents
(e.g.,
browsers) and programs whose purpose is to create style sheets (e.g., editors). A UA may
fall into both categories. (There are other programs that read or write style sheets, but
this module gives no rules for them.)
document
A tree-structured document with elements and attributes, such as an SGML or XML document
[XML11]
style sheet
CSS style sheet
Appendix D — Acknowledgements
The editors would like to thank the members of the W3C Voice Browser
and Cascading Style Sheets working groups
for their assistance in preparing this specification.
Special thanks to Ellen Eide (IBM) for her detailed comments,
and to Elika Etemad (Fantasai) for her thorough reviews.
Appendix E — Changes
The following changes have been made since the
2012 Candidate Recommendation
Renamed the
none
and
normal
values of
speak
to
never
and
always
for clarity.
See
Issue 510
Made the
auto
value of
speak
respond to
visibility
See
Issue 511
In addition there have been some minor editorial fixes.
and the source has been converted to
Bikeshed
format.
Privacy Considerations
No new privacy considerations have been reported on this specification.
Security Considerations
No new security considerations have been reported on this specification.
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

, 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
absolute
value for voice-pitch
, in § 11.3
value for voice-range
, in § 11.4

, in § 11.1
always
, in § 7.1
aural box model
, in § 5
auto
value for speak
, in § 7.1
value for voice-duration
, in § 12.1
center
, in § 6.2
child
, in § 11.1
cue
, in § 10.3
cue-after
, in § 10.1
cue-before
, in § 10.1

type for voice-volume
, in § 6.1
value for voice-volume
, in § 6.1
digits
, in § 7.2
document
, in § 15
fast
, in § 11.2
female
, in § 11.1

value for voice-pitch
, in § 11.3
value for voice-range
, in § 11.4

, in § 11.1

, in § 11.1
high
value for voice-pitch
, in § 11.3
value for voice-range
, in § 11.4

, in § 11.1
left
, in § 6.2
leftwards
, in § 6.2
literal-punctuation
, in § 7.2
loud
, in § 6.1
low
value for voice-pitch
, in § 11.3
value for voice-range
, in § 11.4
male
, in § 11.1
medium
value for pause-before, pause-after
, in § 8.1
value for rest-before, rest-after
, in § 9.1
value for voice-pitch
, in § 11.3
value for voice-range
, in § 11.4
value for voice-rate
, in § 11.2
value for voice-volume
, in § 6.1
moderate
, in § 11.5
neutral
, in § 11.1
never
, in § 7.1
none
value for pause-before, pause-after
, in § 8.1
value for rest-before, rest-after
, in § 9.1
value for voice-stress
, in § 11.5
no-punctuation
, in § 7.2
normal
value for speak-as
, in § 7.2
value for voice-rate
, in § 11.2
value for voice-stress
, in § 11.5

, in § 6.2
old
, in § 11.1
pause
, in § 8.2
pause-after
, in § 8.1
pause-before
, in § 8.1

value for voice-pitch
, in § 11.3
value for voice-range
, in § 11.4

, in § 11.2
preserve
, in § 11.1
reduced
, in § 11.5
rest
, in § 9.2
rest-after
, in § 9.1
rest-before
, in § 9.1
right
, in § 6.2
rightwards
, in § 6.2
semitone
, in § 11.3

type for voice-pitch
, in § 11.3
value for voice-pitch
, in § 11.3
value for voice-range
, in § 11.4
silent
, in § 6.1
slow
, in § 11.2
soft
, in § 6.1
speak
, in § 7.1
speak-as
, in § 7.2
spell-out
, in § 7.2
strong
value for pause-before, pause-after
, in § 8.1
value for rest-before, rest-after
, in § 9.1
value for voice-stress
, in § 11.5
style sheet
, in § 15

value for pause-before, pause-after
, in § 8.1
value for rest-before, rest-after
, in § 9.1
value for voice-duration
, in § 12.1
UA
, in § 15

, in § 10.1
user agent
, in § 15
voice-balance
, in § 6.2
voice-duration
, in § 12.1
voice-family
, in § 11.1

, in § 11.1
voice-pitch
, in § 11.3
voice-range
, in § 11.4
voice-rate
, in § 11.2
voice-stress
, in § 11.5
voice-volume
, in § 6.1
weak
value for pause-before, pause-after
, in § 8.1
value for rest-before, rest-after
, in § 9.1
x-fast
, in § 11.2
x-high
value for voice-pitch
, in § 11.3
value for voice-range
, in § 11.4
x-loud
, in § 6.1
x-low
value for voice-pitch
, in § 11.3
value for voice-range
, in § 11.4
x-slow
, in § 11.2
x-soft
, in § 6.1
x-strong
value for pause-before, pause-after
, in § 8.1
value for rest-before, rest-after
, in § 9.1
x-weak
value for pause-before, pause-after
, in § 8.1
value for rest-before, rest-after
, in § 9.1
young
, in § 11.1
Terms defined by reference
[CSS-BACKGROUNDS-3]
defines the following terms:
border
[CSS-BOX-4]
defines the following terms:
margin
padding
[CSS-CASCADE-5]
defines the following terms:
inherit
[CSS-COUNTER-STYLES-3]
defines the following terms:
armenian
decimal
decimal-leading-zero
georgian
lower-alpha
lower-greek
lower-latin
lower-roman
upper-alpha
upper-latin
upper-roman
[CSS-DISPLAY-4]
defines the following terms:
display
none
visibility
visible
[CSS-FONTS-4]
defines the following terms:
font-family
[CSS-POSITION-3]
defines the following terms:
left
right
[CSS-PSEUDO-4]
defines the following terms:
::after
::before
[CSS-VALUES-4]
defines the following terms:
&&

CSS identifier
CSS-wide keywords
dimension
number
percentage
||
[CSS2]
defines the following terms:

all
screen
speech
[CSS3GENCON]
defines the following terms:
content
[CSS3LIST]
defines the following terms:
list-style-image
list-style-type
[HTML]
defines the following terms:
span
[INFRA]
defines the following terms:
string
[MEDIAQUERIES-5]
defines the following terms:
aural
References
Normative References
[CSS-BACKGROUNDS-3]
Elika Etemad; Brad Kemper.
CSS Backgrounds and Borders Module Level 3
. 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-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-FONTS-4]
Chris Lilley.
CSS Fonts Module Level 4
. URL:
[CSS-POSITION-3]
Elika Etemad; Tab Atkins Jr..
CSS Positioned Layout Module Level 3
. URL:
[CSS-PSEUDO-4]
Elika Etemad; Alan Stearns.
CSS Pseudo-Elements Module Level 4
. URL:
[CSS-VALUES-3]
Tab Atkins Jr.; Elika Etemad.
CSS Values and Units Module Level 3
. URL:
[CSS-VALUES-4]
Tab Atkins Jr.; Elika Etemad.
CSS Values and Units Module Level 4
. URL:
[CSS2]
Bert Bos; et al.
Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification
. URL:
[CSS3GENCON]
Elika Etemad; Mike Bremford.
CSS Generated Content Module Level 3
. URL:
[CSS3LIST]
Elika Etemad; Tab Atkins Jr..
CSS Lists and Counters Module Level 3
. 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:
[SSML]
Daniel Burnett; Zhi Wei Shuang.
Speech Synthesis Markup Language (SSML) Version 1.1
. 7 September 2010. REC. URL:
[XML11]
Tim Bray; et al.
Extensible Markup Language (XML) 1.1 (Second Edition)
. 16 August 2006. REC. URL:
Non-Normative References
[HTML]
Anne van Kesteren; et al.
HTML Standard
. Living Standard. URL:
[PRONUNCIATION-LEXICON]
Paolo Baggia.
Pronunciation Lexicon Specification (PLS) Version 1.0
. 14 October 2008. REC. URL:
[SSML-SAYAS]
Daniel Burnett; et al.
SSML 1.0 say-as attribute values
. 26 May 2005. NOTE. URL:
Property Index
Name
Value
Initial
Applies to
Inh.
%ages
Animation type
Canonical order
Computed value
cue
<'cue-before'> <'cue-after'>?
see individual properties
all elements
no
N/A
see individual properties
per grammar
see individual properties
cue-after
? | none
none
all elements
no
N/A
by computed value type
per grammar
specified value
cue-before
? | none
none
all elements
no
N/A
by computed value type
per grammar
specified value
pause
<'pause-before'> <'pause-after'>?
see individual properties
all elements
no
N/A
see individual properties
per grammar
see individual properties
pause-after
| none | x-weak | weak | medium | strong | x-strong
none
all elements
no
N/A
by computed value type
per grammar
specified value
pause-before
| none | x-weak | weak | medium | strong | x-strong
none
all elements
no
N/A
by computed value type
per grammar
specified value
rest
<'rest-before'> <'rest-after'>?
see individual properties
all elements
no
N/A
see individual properties
per grammar
see individual properties
rest-after
| none | x-weak | weak | medium | strong | x-strong
none
all elements
no
N/A
by computed value type
per grammar
specified value
rest-before
| none | x-weak | weak | medium | strong | x-strong
none
all elements
no
N/A
by computed value type
per grammar
specified value
speak
auto | never | always
auto
all elements
yes
N/A
discrete
per grammar
specified value
speak-as
normal | spell-out || digits || [ literal-punctuation | no-punctuation ]
normal
all elements
yes
N/A
discrete
per grammar
specified value
voice-balance
| left | center | right | leftwards | rightwards
center
all elements
yes
N/A
by computed value type
per grammar
the specified value resolved to a between -100 and 100 (inclusive)
voice-duration
auto |
auto
all elements
no
N/A
by computed value type
per grammar
specified value
voice-family
[ | ]# | preserve
implementation-dependent
all elements
yes
N/A
discrete
per grammar
specified value
voice-pitch
&& absolute | [ [ x-low | low | medium | high | x-high ] ||
[ | | ] ]
medium
all elements
yes
refer to inherited value
by computed value type
per grammar
one of the predefined pitch keywords if only the keyword is specified by itself,
otherwise an absolute frequency calculated by converting the keyword value (if any) to a
fixed frequency based on the current voice-family and by applying the specified relative
offset (if any)
voice-range
&& absolute | [ [ x-low | low | medium | high | x-high ] ||
[ | | ] ]
medium
all elements
yes
refer to inherited value
by computed value type
per grammar
one of the predefined pitch keywords if only the keyword is specified by itself,
otherwise an absolute frequency calculated by converting the keyword value (if any) to a
fixed frequency based on the current voice-family and by applying the specified relative
offset (if any)
voice-rate
[ normal | x-slow | slow | medium | fast | x-fast ] ||
normal
all elements
yes
refer to default value
by computed value type
per grammar
a keyword value, and optionally also a percentage relative to the keyword (if not 100%)
voice-stress
normal | strong | moderate | none | reduced
normal
all elements
yes
N/A
discrete
per grammar
specified value
voice-volume
silent | [ [ x-soft | soft | medium | loud | x-loud ] || ]
medium
all elements
yes
N/A
by computed value type
per grammar
silent, or a keyword value and optionally also a decibel offset (if not zero)