Overview.bs

<pre class='metadata'>
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 <a href="http://www.w3.org/Voice/">Voice Browser Activity</a>.
At Risk: 'voice-balance', 'voice-duration', 'voice-pitch', 'voice-range', and 'voice-stress'
</pre>
<pre class=link-defaults>
spec:html; type:element; text:link
</pre>

<h2 id="intro">
Introduction, design goals</h2>


	<em>This section is non-normative.</em>

	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 <em>listening</em> to content
	(as opposed to <em>reading</em>)
	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.

<h2 id="background">
Background information, CSS 2.1</h2>


	<em>This section is non-normative.</em>

	The CSS Speech module is a re-work of the informative
	<a href="https://www.w3.org/TR/CSS2/aural.html">CSS2.1 Aural appendix</a>,
	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.

<h2 id="ssml-rel">
Relationship with SSML</h2>


	<em>This section is non-normative.</em>

	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.

<h3 id="values">
Value Definitions</h3>


	This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]]
	using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].
	Value types not defined in this specification are defined in CSS Values &amp; 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 <a>CSS-wide keywords</a> keywords as their property value.
	For readability they have not been repeated explicitly.

<h2 id="example">
Example</h2>


	<div class="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 <code>heidi</code>
		are rendered on the left audio channel (and with a female voice, etc.),
		whilst the class <code>peter</code>
		corresponds to the right channel (and to a male voice, etc.).
		The volume level of text spans marked with the class <code>special</code>
		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).

	<pre>
		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;
		}

		...

		&lt;h1&gt;I am Paul, and I speak headings.&lt;/h1&gt;
		&lt;p class="heidi"&gt;Hello, I am Heidi.&lt;/p&gt;
		&lt;p class="peter"&gt;
			&lt;span class="special"&gt;Can you hear me ?&lt;/span&gt;
			I am Peter.
		&lt;/p&gt;</pre>
	</div>

<h2 id="aural-model">
The aural formatting model</h2>


	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 <dfn export lt="aural box model">aural “box” model</dfn>.
	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 &lt;element&gt;:


	<img
		title="The aural 'box' model, illustrated by a diagram: the selected element is positioned in the center, on its left side are (from innermost to outermost) rest-before, cue-before, pause-before, on its right side are (from innermost to outermost) rest-after, cue-after, pause-after, where rest is conceptually similar to padding, cue is similar to border, pause is similar to margin."
		alt="The aural 'box' model, illustrated by a diagram: the selected element is positioned in the center, on its left side are (from innermost to outermost) rest-before, cue-before, pause-before, on its right side are (from innermost to outermost) rest-after, cue-after, pause-after, where rest is conceptually similar to padding, cue is similar to border, pause is similar to margin."
		id="aural-box" src="aural-box.png">

<h2 id="mixing-props">
Mixing properties</h2>


<h3 id="mixing-props-voice-volume">
The 'voice-volume' property</h3>


	<pre class=propdef>
	Name: voice-volume
	Value: silent | [[x-soft | soft | medium | loud | x-loud] || <<decibel>>]
	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)
	</pre>

	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 <a href="#cue-props">audio cues</a>
	within the [=aural box model=] of the selected element.

	Note: Although the functionality provided by this property is similar to
	the <a href="https://www.w3.org/TR/speech-synthesis11/#edef_prosody"><code>volume</code>
	attribute of the <code>prosody</code> element</a> 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.

	<dl dfn-type=value dfn-for=voice-volume>
	<!--<dt><dfn>normal</dfn>
		<dd>
			Corresponds to +0.0dB,
			which means that there is no modification of volume level.
			This value overrides the inherited value. -->

		<dt><dfn>silent</dfn>
		<dd>
			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).

	<dt><dfn>x-soft</dfn>, <dfn>soft</dfn>, <dfn>medium</dfn>, <dfn>loud</dfn>, <dfn>x-loud</dfn>
	<dd>
		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 <em>minimum audible</em> volume level,
		''x-loud'' maps to the user's <em>maximum tolerable</em> volume level,
		''voice-volume/medium'' maps to the user's <em>preferred</em> volume level,
		''soft'' and ''loud'' map to intermediary values.

	<dt><dfn><<decibel>></dfn>
	<dd>
		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 <<decibel>> value.

		The <dfn type><<decibel>></dfn> type denotes
		a [=dimension=] with a "dB" (decibel unit) unit identifier.
		Decibels represent
		the ratio of the squares of the new signal amplitude <var>a1</var>
		and the current amplitude <var>a0</var>,
		as per the following logarithmic equation:
		volume(dB) = 20 &times; log10(<var>a1</var> / <var>a0</var>).

		Note: -6.0dB is approximately half the amplitude of the audio signal,
		and +6.0dB is approximately twice the amplitude.
	</dl>

	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).

<h3 id="mixing-props-voice-balance">
The 'voice-balance' property</h3>


	<pre class=propdef>
	Name: voice-balance
	Value: <<number>> | left | center | right | leftwards | rightwards
	Initial: center
	Applies to: all elements
	Inherited: yes
	Percentages: N/A
	Computed value: the specified value resolved to a &lt;number&gt; between ''-100'' and ''100'' (inclusive)
	</pre>

	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]].

	<dl dfn-type=value dfn-for=voice-balance>
		<dt><dfn><<number>></dfn>
		<dd>
			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).

		<dt><dfn>left</dfn>
		<dd>
			Same as ''-100''.

		<dt><dfn>center</dfn>
		<dd>
			Same as ''0''.

		<dt><dfn>right</dfn>
		<dd>
			Same as ''100''.

		<dt><dfn>leftwards</dfn>
		<dd>
			Moves the sound to the left
			by subtracting 20 from the inherited 'voice-balance' value
			(and by clamping the resulting number to ''-100'').

		<dt><dfn>rightwards</dfn>

		<dd>
			Moves the sound to the right,
			by adding 20 to the inherited 'voice-balance' value
			(and by clamping the resulting number to ''100'').
	</dl>

	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)

<h2 id="speaking-props">
Speaking properties</h2>


<h3 id="speaking-props-speak">
The 'speak' property</h3>


	<pre class=propdef>
	Name: speak
	Value: auto | never | always
	Initial: auto
	Applies to: all elements
	Inherited: yes
	Percentages: N/A
	Computed value: specified value
	</pre>

	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]].

	<dl dfn-type=value dfn-for=speak>
	<dt><dfn>auto</dfn>
	<dd>
		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''.

	<dt><dfn>never</dfn>
	<dd>
		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 <a href="#collapsed-pauses">collapsing of pauses</a>
		or additive behavior of adjoining rests.

	<dt><dfn>always</dfn>
	<dd>
		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.

	</dl>

<h3 id="speaking-props-speak-as">
The 'speak-as' property</h3>

	<pre class=propdef>
		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
	</pre>

	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 <a href="https://www.w3.org/TR/speech-synthesis11/#edef_say-as"><code>say-as</code> element</a>
	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.

	<dl dfn-type=value dfn-for=speak-as>
		<dt><dfn>normal</dfn>
		<dd>
			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.

		<dt><dfn>spell-out</dfn>
		<dd>
			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 “r&ocirc;le” can also be written as “role”.
			A conforming implementation would thus be able to spell-out “r&ocirc;le” as “R O L E”.

		<dt><dfn>digits</dfn>
		<dd>
			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 <em>number</em> 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.

		<dt><dfn>literal-punctuation</dfn>
		<dd>
			Punctuation such as semicolons, braces, and so on
			is named aloud (i.e. spoken literally)
			rather than rendered naturally as appropriate pauses.

		<dt><dfn>no-punctuation<d/fn>
		<dd>
			Punctuation is not rendered: neither spoken nor rendered as pauses.

	</dl>

<h2 id="pause-props">
Pause properties </h2>


<h3 id="pause-props-pause-before-after">
The 'pause-before' and 'pause-after' properties</h3>


	<pre class=propdef>
	Name: pause-before, pause-after
	Value: &lt;time&gt; | none | x-weak | weak | medium | strong | x-strong
	Initial: none
	Applies to: all elements
	Inherited: no
	Percentages: N/A
	Computed value: specified value
	</pre>

	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 <a href="https://www.w3.org/TR/speech-synthesis11/#edef_break"><code>break</code> element</a>
	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. <a href="#collapsed-pauses">"collapsed" pauses</a>).

	<dl dfn-type=value dfn-for="pause-before,pause-after">
		<dt><dfn><<time>></dfn>
		<dd>
			Expresses the pause in absolute time units
			(seconds and milliseconds, e.g. "+3s", "250ms").
			Only non-negative values are allowed.

		<dt><dfn>none</dfn>
		<dd>
			Equivalent to 0ms (no prosodic break is produced by the speech processor).

		<dt><dfn>x-weak</dfn>, <dfn>weak</dfn>, <dfn>medium</dfn>, <dfn>strong</dfn>, and <dfn>x-strong</dfn>

		<dd>
			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.

	</dl>

	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.

	<div class="example">
		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.
		<pre>
			p { pause: none } /* pause-before: none; pause-after: none */
		</pre>
	</div>

<h3 id="pause-props-pause">
The 'pause' shorthand property</h3>

	<pre class=propdef>
	Name: pause
	Value: &lt;'pause-before'&gt; &lt;'pause-after'&gt;?
	Initial: N/A (see individual properties)
	Applies to: all elements
	Inherited: no
	Percentages: N/A
	Computed value: N/A (see individual properties)
	</pre>

	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.

	<div class="example">
		<p> Examples of property values:
		<pre>
			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: <i>unspecified</i>; pause-after: 10ms */
		</pre>
	</div>

<h3 id="collapsed-pauses">
Collapsing pauses</h3>

	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:
	<ul>
		<li>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'.
		<li>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'.
		<li>The 'pause-after' of an aural "box" and the 'pause-before' of its next sibling.
		<li>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').
	</ul>

	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]].

<h2 id="rest-props">
Rest properties</h2>

<h3 id="rest-props-rest-before-after">
The 'rest-before' and 'rest-after' properties</h3>

	<pre class=propdef>
	Name: rest-before, rest-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
	</pre>

	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 <a href="https://www.w3.org/TR/speech-synthesis11/#edef_break"><code>break</code> element</a>
	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).

	<dl dfn-type=value dfn-for="rest-before,rest-after">
		<dt><dfn><<time>></dfn>
		<dd>
			Expresses the rest in absolute time units (seconds and milliseconds, e.g. "+3s", "250ms").
			Only non-negative values are allowed.

		<dt><dfn>none</dfn>
		<dd>
			Equivalent to 0ms.
			(No prosodic break is produced by the speech processor.)

		<dt><dfn>x-weak</dfn>, <dfn>weak</dfn>, <dfn>medium</dfn>, <dfn>strong</dfn>, and <dfn>x-strong</dfn>
		<dd>
			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.
	</dl>

	As opposed to <a href="#pause-props">pause properties</a>,
	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.

<h3 id="rest-props-rest">
The 'rest' shorthand property</h3>

	<pre class=propdef>
	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)
	</pre>

	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.

<h2 id="cue-props">
Cue properties</h2>

<h3 id="cue-props-cue-before-after">
The 'cue-before' and 'cue-after' properties</h3>

	<pre class=propdef>
	Name: cue-before, cue-after
	Value: <<uri>> <<decibel>>? | none
	Initial: none
	Applies to: all elements
	Inherited: no
	Percentages: N/A
	Computed value: specified value
	</pre>

	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 <a href="#aural-model">aural box model</a>.

	Note: Although the functionality provided by this property may appear related to
	the <a href="https://www.w3.org/TR/speech-synthesis11/#edef_audio"><code>audio</code> element</a>
	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 <code>audio</code> element.

	<dl dfn-type=value dfn-for="cue-before,cue-after">
		<dt><dfn><<uri>></dfn>
		<dd>
			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.

		<dt>none
		<dd>
			Specifies that no auditory icon is used.

		<dt><<decibel>>
		<dd>
			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 <<decibel>> 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).
	</dl>

	<div class="example">
		<p> Examples of property values:
		<pre>
			a
			{
			  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; }
		</pre>
	</div>

<h3 id="cue-props-volume">
Relation between audio cues and speech synthesis volume levels</h3>


	<em>This section is non-normative.</em>

	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 <<decibel>> 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 <<decibel>> 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.

<h3 id="cue-props-cue">
The 'cue' shorthand property</h3>

	<pre class=propdef>
	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)
	</pre>

	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.

	<div class="example">
		<p> Example of shorthand notation:
		<pre>
			h1
			{
			  cue-before: url(pop.au);
			  cue-after: url(pop.au);
			}
			/* ...is equivalent to: */
			h1
			{
			  cue: url(pop.au);
			}
		</pre>
	</div>

<h2 id="voice-char-props">
Voice characteristic properties</h2>


<h3 id="voice-props-voice-family">
The 'voice-family' property</h3>

	<pre class=propdef>
	Name: voice-family
	Value:  [[&lt;family-name&gt; | &lt;generic-voice&gt;],]* [&lt;family-name&gt; | &lt;generic-voice&gt;] | preserve
	Initial: implementation-dependent
	Applies to: all elements
	Inherited: yes
	Percentages: N/A
	Computed value: specified value
	</pre>

	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 <a href="#voice-selection">voice selection</a> section on this topic.

	<dfn><<generic-voice>></dfn> = [<<age>>? <<gender>> <<integer>>?]

	Note: Although the functionality provided by this property is similar to
	the <a href="https://www.w3.org/TR/speech-synthesis11/#edef_voice"><code>voice</code> element</a>
	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.

	<dl dfn-type=value dfn-for=voice-family>
		<dt><dfn><<family-name>></dfn>
		<dd>
			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 <css>default</css> 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: <code>voice-family: "john doe", "Henry	the-8th";</code>

		<dt><dfn type><<age>></dfn>
		<dd>
			Possible values are <dfn>child</dfn>, <dfn>young</dfn> and <dfn>old</dfn>,
			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.

		<d><dfn type><<gender>></dfn>
		<dd>
			One of the keywords <dfn>male</dfn>, <dfn>female</dfn>, or <dfn>neutral</dfn>,
			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.

		<dt><dfn><<integer>></dfn>
		<dd>
			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.

		<dt><dfn>preserve</dfn>
		<dd>
			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).
	</dl>

	<div class="example">
		<p> Examples of invalid declarations:
		<pre>
			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 */
		</pre>
	</div>

<h4 class="no-toc" id="voice-selection">
Voice selection, content language</h4>

	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):

	<ul>
	<li>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.
	<li>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.
	<li>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.
	</ul>

	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.

	<div class="example">
		<p>Examples of property values:
		<pre>
			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; }

			...

			&lt;p class="romeo" xml:lang="en-US"&gt;
			  The French text below will be spoken with an English voice:
			  &lt;span style="voice-family: preserve;" xml:lang="fr-FR"&gt;Bonjour monsieur !&lt;/span&gt;

			  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):
			  &lt;span style="voice-family: female;"&gt;Hello sir!&lt;/span&gt;
			&lt;/p&gt;
		</pre>
	</div>

<h3 id="voice-props-voice-rate">
The 'voice-rate' property</h3>

	<pre class=propdef>
	Name: voice-rate
	Value: [normal | x-slow | slow | medium | fast | x-fast] || <<percentage>>
	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%)
	</pre>

	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 <a href="https://www.w3.org/TR/speech-synthesis11/#edef_prosody"><code>rate</code> attribute
	of the <code>prosody</code> element</a> 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.

	<dl dfn-type=value dfn-for=voice-rate>
		<dt><dfn>normal</dfn>
		<dd>
			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.

		<dt><dfn>x-slow</dfn>, <dfn>slow</dfn>, <dfn>medium</dfn>, <dfn>fast</dfn> and <dfn>x-fast</dfn>
		<dd>
			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.

		<dt><dfn><<percentage>></dfn>
		<dd>
			<p>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.
	</dl>

	<div class="example">
		<p>Examples of inherited values:
		<pre>
			&lt;body&gt;
			  &lt;e1&gt;
			    &lt;e2&gt;
			      &lt;e3&gt;
			        ...
			      &lt;/e3&gt;
			    &lt;/e2&gt;
			  &lt;/e1&gt;
			&lt;/body&gt;

			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) */
		</pre>
	</div>

<h3 id="voice-props-voice-pitch">
The 'voice-pitch' property</h3>

	<pre class=propdef>
	Name: voice-pitch
	Value: <<frequency>> && absolute | [[x-low | low | medium | high | x-high] || [<<frequency>> | <<semitones>> | <<percentage>>]]
	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)
	</pre>

	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 <a href="https://www.w3.org/TR/speech-synthesis11/#edef_prosody"><code>pitch</code>
	attribute of the <code>prosody</code> element</a> 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.

	<dl dfn-type=value dfn-for=voice-pitche>
		<dt><dfn><<frequency>></dfn>
		<dd>
			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'').

		<dt><dfn>absolute</dfn>
		<dd>
			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.

		<dt><dfn><<semitones>></dfn>
		<dd>
			Specifies a relative change (decrement or increment) to the inherited value.
			The syntax of <dfn type><<semitones>></dfn> allowed values is
			a [=dimension=] with the unit identifier ''st'' (semitones).
			A <dfn dfn>semitone</dfn> 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.)

		<dt><dfn><<percentage>></dfn>
		<dd>
			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 <code>200 + (200*0.5)</code> = 300Hz.
			Conversely, -50% results in <code>200-(200*0.5)</code> = 100Hz.

		<dt><dfn>x-low</dfn>, <dfn>low</dfn>, <dfn>medium</dfn>, <dfn>high</dfn>, <dfn>x-high</dfn>
		<dd>
			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.
	</dl>

	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.

	<div class="example">
		<p>Examples of property values:
		<pre>
			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 */
		</pre>
	</div>

<h3 id="voice-props-voice-range">
The 'voice-range' property</h3>

	<pre class=propdef>
	Name: voice-range
	Value: <<frequency>> && absolute | [[x-low | low | medium | high | x-high] || [<<frequency>> | <<semitones>> | <<percentage>>]]
	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)
	</pre>

	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 <a href="https://www.w3.org/TR/speech-synthesis11/#edef_prosody"><code>range</code>
		attribute of the <code>prosody</code> element</a> 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.

	<dl dfn-type=value dfn-for=voice-range>
		<dt><<frequency>>
		<dd>
			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'').

		<dt><dfn>absolute</dfn>
		<dd>
			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.

		<dt><dfn><<semitones>></dfn>
		<dd>
			Specifies a relative change (decrement or increment) to the inherited value
			as a [=semitone=].

		<dt><dfn><<percentage>></dfn>
		<dd>
			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 <code>200 + (200*0.5)</code> = 300Hz.
			Conversely, -50% results in <code>200-(200*0.5)</code> = 100Hz.

		<dt><dfn>x-low</dfn>, <dfn>low</dfn>, <dfn>medium</dfn>, <dfn>high</dfn>, <dfn>x-high</dfn>
		<dd>
			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.

	</dl>

	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.

	<div class="example">
		<p>Examples of inherited values:
		<pre>
			&lt;body&gt;
			  &lt;e1&gt;
			    &lt;e2&gt;
			      &lt;e3&gt;
			        &lt;e4&gt;
			          &lt;e5&gt;
			            &lt;e6&gt;
			              ...
			            &lt;/e6&gt;
			          &lt;/e5&gt;
			        &lt;/e4&gt;
			      &lt;/e3&gt;
			    &lt;/e2&gt;
			  &lt;/e1&gt;
			&lt;/body&gt;



			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) */
		</pre>
	</div>
<h3 id="voice-props-voice-stress">
The 'voice-stress' property</h3>

	<pre class=propdef>
	Name: voice-stress
	Value: normal | strong | moderate | none | reduced
	Initial: normal
	Applies to: all elements
	Inherited: yes
	Percentages: N/A
	Computed value: specified value
	</pre>

	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 <a href="https://www.w3.org/TR/speech-synthesis11/#edef_emphasis"><code>emphasis</code>
	element</a> from the SSML markup language [[!SSML]].

	<dl dfn-type=value dfn-for=voice-stress>
		<dt><dfn>normal</dfn>
		<dd>
			Represents the default emphasis produced by the speech synthesizer.

		<dt><dfn>none</dfn>
		<dd>
			<p>Prevents the synthesizer from emphasizing text it would normally emphasize.

		<dt><dfn>moderate</dfn> and <dfn>strong</dfn>
		<dd>
			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'').

		<dt><dfn>reduced</dfn>
		<dd>
			Effectively the opposite of emphasizing a word.

	</dl>

	<div class="example">
		<p>Examples of property values, with HTML sample:
		<pre>
			.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; }

			...

			&lt;p&gt;This is a big car.&lt;/p&gt;
			&lt;!-- The speech output from the line above is identical to the line below: --&gt;
			&lt;p&gt;This is a &lt;em class="default-emphasis"&gt;big&lt;/em&gt; car.&lt;/p&gt;

			&lt;p&gt;This car is &lt;em class="lowered-emphasis"&gt;massive&lt;/em&gt;!&lt;/p&gt;
			&lt;!-- The "em" below is totally de-emphasized, whereas the emphasis in the line above is only reduced: --&gt;
			&lt;p&gt;This car is &lt;em class="removed-emphasis"&gt;massive&lt;/em&gt;!&lt;/p&gt;

			&lt;!-- The lines below demonstrate increasing levels of emphasis: --&gt;
			&lt;p&gt;This is a &lt;em class="normal-emphasis"&gt;big&lt;/em&gt; car!&lt;/p&gt;
			&lt;p&gt;This is a &lt;em class="huge-emphasis"&gt;big&lt;/em&gt; car!!!&lt;/p&gt;
		</pre>
	</div>

<h2 id="duration-props">
Voice duration property</h2>

<h3 id="mixing-props-voice-duration">
The 'voice-duration' property</h3>

	<pre class=propdef>
	Name: voice-duration
	Value: auto | &lt;time&gt;
	Initial: auto
	Applies to: all elements
	Inherited: no
	Percentages: N/A
	Computed value: specified value
	</pre>

	The 'voice-duration' property specifies how long it should take to render
	the selected element's content
	(not including <a href="#cue-props">audio cues</a>, <a href="#pause-props">pauses</a> and <a href="#rest-props">rests</a> ).
	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 <<time>> 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 <a href="https://www.w3.org/TR/speech-synthesis11/#edef_prosody"><code>duration</code> attribute
	of the <code>prosody</code> element</a> from the SSML markup language [[!SSML]].

	<dl dfn-type=value dfn-for=voice-duration>
		<dt><dfn>auto</dfn>
		<dd>
			Resolves to a used value corresponding to the duration of the speech synthesis
			when using the inherited 'voice-rate'.

		<dt><dfn><<time>></dfn>
		<dd>
			Specifies a value in absolute time units
			(seconds and milliseconds, e.g. "+3s", "250ms").
			Only non-negative values are allowed.

	</dl>

<h2 id="lists">
List items and counters styles</h2>

	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.

	<dl>
		<dt>''list-style/disc'', ''list-style/circle'', ''list-style/square''
		<dd>
			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.

		<dt>''decimal'', ''decimal-leading-zero'', ''lower-roman'', ''upper-roman'', ''georgian'', ''armenian''
		<dd>
			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.

		<dt>''lower-latin'', ''lower-alpha'', ''upper-latin'', ''upper-alpha'', ''lower-greek''
		<dd>
			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)
	</dl>

	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).

<h2 id="content">
Inserted and replaced content</h2>

	<em>This section is non-normative.</em>

	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 <a href="https://www.w3.org/TR/speech-synthesis11/#edef_sub"><code>alias</code> attribute
	of the <code>sub</code> element</a> from the SSML markup language [[!SSML]].

	<div class="example">
		In this example, the abbreviation is rendered
		using the content of the title attribute instead of the element's content.

		<pre>
			/* This replaces the content of the selected element
			by the string "World Wide Web Consortium". */
			abbr { content: attr(title); }
			...

			&lt;abbr title="World Wide Web Consortium"&gt;W3C&lt;/abbr&gt;
		</pre>
	</div>

	In a similar way, text strings in a document can be replaced by a previously recorded version.
	<div class="example">
		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.
		<pre>
			.hamlet { content: url(./audio/gielgud.wav); }
			...

			&lt;div class="hamlet"&gt;
			To be, or not to be: that is the question:
			&lt;/div&gt;</pre>
	</div>

	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.

	<div class="example">
		<p>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.
		<pre>
			ul::before { content: "Start list: "; }
			ul::after  { content: "List end. "; }
			li::before { content: "List item: "; }</pre>
	</div>

	Detailed information can be found in the CSS3 Generated and Replaced Content module
	[[CSS3GENCON]].

<h2 id="pronunciation">
 Pronunciation, phonemes </h2>

	<em>This section is non-normative.</em>

	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 <a href="http://microformats.org/wiki/rel-pronunciation">"pronunciation"</a>
	<code>rel</code> value allows importing pronunciation lexicons in HTML documents
	using the <code>link</code> 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 <a href="http://idpf.org/epub/30">EPUB 3.0 specification</a> allows
	(x)HTML5 documents to contain attributes derived from the [[!SSML]] specification,
	that describe how to pronounce text based on a particular phonetic alphabet.

	<!-- p>
	One avenue to explore is the use CSS to "bind" HTML text with a
	phoneme (also declared in the HTML document). This would maintain a
	clear separation between content and presentation, and it would allow
	authors to define different pronunciations for one given text token
	(Media Queries could drive the switch of stylesheet to import). This
	possibility has been mentioned several times by Working Group members
	as well as people from the public mailing-list, so it cannot be
	ignored. However, there are architectural considerations (e.g.
	collision between CSS versus HTML -defined phonemes) which make this a
	lot trickier to standardize than it sounds. The
	whole "speech synthesis" issue should be tackled globally at the level
	of the W3C ecosystem. For example, there are many cross-cutting
	concerns with the work done by the HTML-Audio and HTML-Speech
	Incubator Groups.
	</p -->

<h3 class="no-num" id="glossary">Glossary</h3>


	The following terms and abbreviations are used in this module.
	<dl>
	<dt>
		<dfn>UA</dfn>

	<dt>
		<dfn id="user-agent">User Agent</dfn>

	<dd>
		<p>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 <span>documents</span> (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.)

	<dt>
		<dfn title="document|documents">document</dfn>

	<dd>
		<p>A tree-structured document with elements and attributes, such as an SGML or XML document
			[[!XML11]].

	<dt>
		<dfn>style sheet</dfn>

	<dd>
		<p>A <a href="https://www.w3.org/TR/CSS2/conform.html#style-sheet">CSS style sheet</a>

	</dl>

	<!-- h3 class="no-num" id="levels">Levels</h3>


		<p><em>This section is informative.</em> CSS has different levels of
		features, each a subset of the other. (See [[CSSBEIJING]] for a full
		explanation.) The lists below describe which features from this
		specification are in each level.

		<h4 class="no-num" id="level-1">CSS Level 1</h4>

		<ul>
		<li>'background-color'
		<li>'background-image' only one image (no layers)
		<li>'background-repeat': only 'repeat' | 'repeat-x' | 'repeat-y' | 'no-repeat'
		<li>'background-attachment': only 'scroll' | 'fixed'
		<li>'background-position': only one or two values allowed
		<li>'background' shorthand: only color, image, repeat, attachment and position
		<li>'border-color' properties
		<li>'border-style' properties
		<li>'border-width' properties
		<li>'border-top', 'border-bottom', 'border-right', 'border-left', and 'border' shorthands
		</ul>

		<h4 class="no-num" id="level-2">CSS Level 2</h4>

		<ul>
		<li>'background-color'
		<li>'background-image': only one image (no layers)
		<li>'background-repeat': only 'repeat' | 'repeat-x' | 'repeat-y' | 'no-repeat'
		<li>'background-attachment': only 'scroll' | 'fixed'
		<li>'background-position': only one or two values allowed
		<li>'background': only color, image, repeat, attachment and position
		<li>'border-color' properties
		<li>'border-style' properties
		<li>'border-width' properties
		<li>'border-top', 'border-bottom', 'border-right', 'border-left', and 'border' shorthands
		</ul>

		<h4 class="no-num" id="level-3">CSS Level 3</h4>

		<ul>
		<li>All features described in the CSS3 Speech module
		</ul -->


<h2 class="no-num" id="ack">
Appendix D &mdash; Acknowledgements</h2>

	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.

<h2 class="no-num" id="changes">
Appendix E &mdash; Changes</h2>

	The following changes have been made since the <a href="https://www.w3.org/TR/2012/CR-css3-speech-20120320/">2012 Candidate Recommendation</a>:

	<ul>
	<li>Renamed the <css>none</css> and <css>normal</css> values of 'speak' to ''never'' and ''always'' for clarity.
	See <a href="https://github.com/w3c/csswg-drafts/issues/510">Issue 510</a>.
	<li>Made the ''speak/auto'' value of 'speak' respond to 'visibility'.
	See <a href="https://github.com/w3c/csswg-drafts/issues/511">Issue 511</a>.
	</ul>