Title: CSS Speech Module Level 1
Shortname: css-speech
Level: 1
Group: csswg
Status: ED
Work Status: Testing
ED: https://drafts.csswg.org/css-speech-1/
TR: https://www.w3.org/TR/css-speech-1/
Editor: Léonie Watson, Tetralogical, lwatson@tetralogical.com
Editor: Daniel Weck, DAISY Consortium, dweck@daisy.org
Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
Former Editor: Claudio Santambrogio, Opera Software
Former Editor: Dave Ragett, W3C / Canon, dsr@w3.org
Previous Version: https://www.w3.org/TR/2020/CR-css-speech-1-20200310/
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.
At Risk: 'voice-balance', 'voice-duration', 'voice-pitch', 'voice-range', and 'voice-stress'
spec:html; type:element; text:link
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.
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.
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.
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.
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;
}
...
<h1>I am Paul, and I speak headings.</h1>
<p class="heidi">Hello, I am Heidi.</p>
<p class="peter">
<span class="special">Can you hear me ?</span>
I am Peter.
</p>
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 <element>:
Mixing properties
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)
The 'voice-volume' property allows authors to control
the amplitude of the audio waveform generated by the speech synthesiser,
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 ''speak/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,
''voice-volume/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
a [=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).
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 <number> between ''-100'' and ''100'' (inclusive)
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]].
<>
A [=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 ''0'' 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 ''0''.
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 ''voice-balance/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 ''0'' maps to zero degrees (''voice-balance/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)
Speaking properties
The 'speak' property
Name: speak
Value: auto | never | always
Initial: auto
Applies to: all elements
Inherited: yes
Percentages: N/A
Computed value: specified value
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 ''speak/never''
when 'display' is ''display/none'',
otherwise resolves to a computed value of ''speak/auto''.
The used value of a computed ''speak/auto'' is equivalent
to ''speak/always'' if 'visibility' is ''visibility/visible''
and to ''never'' otherwise.
Note: The ''display/none'' value of the 'display' property
cannot be overridden by descendants of the selected element,
but the ''speak/auto'' value of 'speak' can, however,
be overridden using either of ''speak/never'' or ''speak/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.
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
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 as 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.
Pause properties
The 'pause-before' and 'pause-after' properties
Name: pause-before, pause-after
Value: <time> | none | x-weak | weak | medium | strong | x-strong
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: specified value
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.
Name: pause
Value: <<'pause-before'>> <<'pause-after'>>?
Initial: N/A (see individual properties)
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: N/A (see individual properties)
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.
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 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 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]].
Rest properties
The 'rest-before' and 'rest-after' properties
Name: rest-before, rest-after
Value: <
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.
The 'rest' shorthand property
Name: rest
Value: <<'rest-before'>> <<'rest-after'>>?
Initial: N/A (see individual properties)
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: N/A (see individual properties)
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.
Cue properties
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
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 ''cue-before/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).
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.
The 'cue' shorthand property
Name: cue
Value: <<'cue-before'>> <<'cue-after'>>?
Initial: N/A (see individual properties)
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: N/A (see individual properties)
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.
Name: voice-family
Value: [[<family-name> | <generic-voice>],]* [<family-name> | <generic-voice>] | preserve
Initial: implementation-dependent
Applies to: all elements
Inherited: yes
Percentages: N/A
Computed value: specified value
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 [=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 ''1'' 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 */
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; }
...
<p class="romeo" xml:lang="en-US">
The French text below will be spoken with an English voice:
<span style="voice-family: preserve;" xml:lang="fr-FR">Bonjour monsieur !</span>
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):
<span style="voice-family: female;">Hello sir!</span>
</p>
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%)
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>
<e1>
<e2>
<e3>
...
</e3>
</e2>
</e1>
</body>
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) */
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)
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 ''voice-pitch/absolute'' keyword is specified.
Otherwise (when the ''voice-pitch/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
a [=dimension=] with the unit identifier ''st'' (semitones).
A semitone interval corresponds to the step between each note
on an equal temperament chromatic scale.
A [=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 */
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)
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 ''voice-range/absolute'' keyword is specified.
Otherwise (when the ''voice-range/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>
<e1>
<e2>
<e3>
<e4>
<e5>
<e6>
...
</e6>
</e5>
</e4>
</e3>
</e2>
</e1>
</body>
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) */
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
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 ''voice-stress/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; }
...
<p>This is a big car.</p>
<!-- The speech output from the line above is identical to the line below: -->
<p>This is a <em class="default-emphasis">big</em> car.</p>
<p>This car is <em class="lowered-emphasis">massive</em>!</p>
<!-- The "em" below is totally de-emphasized, whereas the emphasis in the line above is only reduced: -->
<p>This car is <em class="removed-emphasis">massive</em>!</p>
<!-- The lines below demonstrate increasing levels of emphasis: -->
<p>This is a <em class="normal-emphasis">big</em> car!</p>
<p>This is a <em class="huge-emphasis">big</em> car!!!</p>
Voice duration property
The 'voice-duration' property
Name: voice-duration
Value: auto | <time>
Initial: auto
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: specified value
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 ''voice-duration/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 ''voice-duration/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 <
