Aural style sheets

This chapter is informative. UAs are not required to implement the properties of this chapter in order to conform to CSS 2.1.

A.1 The media types 'aural' and 'speech'

We expect that in a future level of CSS there will be new properties and values defined for speech output. Therefore CSS 2.1 reserves the 'speech' media type (see chapter 7, "Media types"), but does not yet define which properties do or do not apply to it.

The properties in this appendix apply to a media type 'aural', that was introduced in CSS2. The type 'aural' is now deprecated.

This means that a style sheet such as

@media speech {
  body { voice-family: Paul }
}

is valid, but that its meaning is not defined by CSS 2.1, while

@media aural {
  body { voice-family: Paul }
}

is deprecated, but defined by this appendix.

A.2 Introduction to aural style sheets

The aural rendering of a document, already commonly used by the blind and print-impaired communities, combines speech synthesis and "auditory icons." Often such aural presentation occurs by converting the document to plain text and feeding this to a screen reader -- software or hardware that simply reads all the characters on the screen. This results in less effective presentation than would be the case if the document structure were retained. Style sheet properties for aural presentation may be used together with visual properties (mixed media) or as an aural alternative to visual presentation.

Besides the obvious accessibility advantages, there are other large markets for listening to information, including in-car use, industrial and medical documentation systems (intranets), home entertainment, and to help users learning to read or who have difficulty reading.

When using aural properties, the canvas consists of a three-dimensional physical space (sound surrounds) and a temporal space (one may specify sounds before, during, and after other sounds). The CSS properties also allow authors to vary the quality of synthesized speech (voice type, frequency, inflection, etc.).

h1, h2, h3, h4, h5, h6 {
    voice-family: paul;
    stress: 20;
    richness: 90;
    cue-before: url("ping.au")
}
p.heidi { azimuth: center-left }
p.peter { azimuth: right }
p.goat  { volume: x-soft }

This will direct the speech synthesizer to speak headers in a voice (a kind of "audio font") called "paul", on a flat tone, but in a very rich voice. Before speaking the headers, a sound sample will be played from the given URL. Paragraphs with class "heidi" will appear to come from front left (if the sound system is capable of spatial audio), and paragraphs of class "peter" from the right. Paragraphs with class "goat" will be very soft.

A.2.1 Angles

Angle values are denoted by <angle> in the text. Their format is a <number> immediately followed by an angle unit identifier.

Angle values may be negative. They should be normalized to the range 0-360deg by the user agent. For example, -10deg and 350deg are equivalent.

Like for <length>, the unit may be omitted, if the value is zero: '0deg' may be written as '0'.

A.2.2 Times

Time values are denoted by <time> in the text. Their format is a <number> immediately followed by a time unit identifier.

Like for <length>, the unit may be omitted, if the value is zero: '0s' may be written as '0'.

A.2.3 Frequencies

Frequency values are denoted by <frequency> in the text. Their format is a <number> immediately followed by a frequency unit identifier.

Like for <length>, the unit may be omitted, if the value is zero: '0Hz' may be written as '0'.

A.3 Volume properties: 'volume'

Volume refers to the median volume of the waveform. In other words, a highly inflected voice at a volume of 50 might peak well above that. The overall values are likely to be human adjustable for comfort, for example with a physical volume control (which would increase both the 0 and 100 values proportionately); what this property does is adjust the dynamic range.

User agents should allow the values corresponding to '0' and '100' to be set by the listener. No one setting is universally applicable; suitable values depend on the equipment in use (speakers, headphones), the environment (in car, home theater, library) and personal preferences. Some examples:

The same author style sheet could be used in all cases, simply by mapping the '0' and '100' points suitably at the client side.

A.4 Speaking properties: 'speak'

This property specifies whether text will be rendered aurally and if so, in what manner. The possible values are:

Note the difference between an element whose 'volume' property has a value of 'silent' and an element whose 'speak' property has the value 'none'. The former takes up the same time as if it had been spoken, including any pause before and after the element, but no sound is generated. The latter requires no time and is not rendered (though its descendants may be).

A.5 Pause properties: 'pause-before', 'pause-after', and 'pause'

These properties specify a pause to be observed before (or after) speaking an element's content. Values have the following meanings:

The pause is inserted between the element's content and any 'cue-before' or 'cue-after' content.

Authors should use relative units to create more robust style sheets in the face of large changes in speech-rate.

A.6 Cue properties: 'cue-before', 'cue-after', and 'cue'

Auditory icons are another way to distinguish semantic elements. Sounds may be played before and/or after the element to delimit it. Values have the following meanings:

The 'cue' property is a shorthand for setting '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.

If a user agent cannot render an auditory icon (e.g., the user's environment does not permit it), we recommend that it produce an alternative cue.

Please see the sections on the :before and :after pseudo-elements for information on other content generation techniques. 'cue-before' sounds and 'pause-before' gaps are inserted before content from the ':before' pseudo-element. Similarly, 'pause-after' gaps and 'cue-after' sounds are inserted after content from the ':after' pseudo-element.

A.7 Mixing properties: 'play-during'

Similar to the 'cue-before' and 'cue-after' properties, this property specifies a sound to be played as a background while an element's content is spoken. Values have the following meanings:

A.8 Spatial properties: 'azimuth' and 'elevation'

Spatial audio is an important stylistic property for aural presentation. It provides a natural way to tell several voices apart, as in real life (people rarely all stand in the same spot in a room). Stereo speakers produce a lateral sound stage. Binaural headphones or the increasingly popular 5-speaker home theater setups can generate full surround sound, and multi-speaker setups can create a true three-dimensional sound stage. VRML 2.0 also includes spatial audio, which implies that in time consumer-priced spatial audio hardware will become more widely available.

This property is most likely to be implemented by mixing the same signal into different channels at differing volumes. It might also use phase shifting, digital delay, and other such techniques to provide the illusion of a sound stage. The precise means used to achieve this effect and the number of speakers used to do so are user agent-dependent; this property merely identifies the desired end result.

If spatial-azimuth is specified and the output device cannot produce sounds behind the listening position, user agents should convert values in the rearwards hemisphere to forwards hemisphere values. One method is as follows:

The precise means used to achieve this effect and the number of speakers used to do so are undefined. This property merely identifies the desired end result.

A.9 Voice characteristic properties: 'speech-rate', 'voice-family', 'pitch', 'pitch-range', 'stress', and 'richness'

This property specifies the speaking rate. Note that both absolute and relative keyword values are allowed (compare with 'font-size'). Values have the following meanings:

The value is a comma-separated, prioritized list of voice family names (compare with 'font-family'). Values have the following meanings:

Names of specific voices may be quoted, and indeed must be quoted if any of the words that make up the name does not conform to the syntax rules for identifiers. It is also recommended to quote specific voices with a name consisting of more than one word. If quoting is omitted, any whitespace characters before and after the voice family name are ignored and any sequence of whitespace characters inside the voice family name is converted to a single space.

Specifies the average pitch (a frequency) of the speaking voice. The average pitch of a voice depends on the voice family. For example, the average pitch for a standard male voice is around 120Hz, but for a female voice, it's around 210Hz.

Specifies variation in average pitch. The perceived pitch of a human voice is determined by the fundamental frequency and typically has a value of 120Hz for a male voice and 210Hz for a female voice. Human languages are spoken with varying inflection and pitch; these variations convey additional meaning and emphasis. Thus, a highly animated voice, i.e., one that is heavily inflected, displays a high pitch range. This property specifies the range over which these variations occur, i.e., how much the fundamental frequency may deviate from the average pitch.

Specifies the height of "local peaks" in the intonation contour of a voice. For example, English is a stressed language, and different parts of a sentence are assigned primary, secondary, or tertiary stress. The value of 'stress' controls the amount of inflection that results from these stress markers. This property is a companion to the 'pitch-range' property and is provided to allow developers to exploit higher-end auditory displays.

Specifies the richness, or brightness, of the speaking voice. A rich voice will "carry" in a large room, a smooth voice will not. (The term "smooth" refers to how the wave form looks when drawn.)

A.10 Speech properties: 'speak-punctuation' and 'speak-numeral'

This property specifies how punctuation is spoken. Values have the following meanings:

This property controls how numerals are spoken. Values have the following meanings:

A.11 Audio rendering of tables

When a table is spoken by a speech generator, the relation between the data cells and the header cells must be expressed in a different way than by horizontal and vertical alignment. Some speech browsers may allow a user to move around in the 2-dimensional space, thus giving them the opportunity to map out the spatially represented relations. When that is not possible, the style sheet must specify at which points the headers are spoken.

A.11.1 Speaking headers: the 'speak-header' property

This property specifies whether table headers are spoken before every cell, or only before a cell when that cell is associated with a different header than the previous cell. Values have the following meanings:

Each document language may have different mechanisms that allow authors to specify headers. For example, in HTML 4 ([HTML4]), it is possible to specify header information with three different attributes ("headers", "scope", and "axis"), and the specification gives an algorithm for determining header information when these attributes have not been specified.

Image of a table created in MS
Word [D]

Image of a table with header cells ("San Jose" and "Seattle") that are not in the same column or row as the data they apply to.

This HTML example presents the money spent on meals, hotels and transport in two locations (San Jose and Seattle) for successive days. Conceptually, you can think of the table in terms of an n-dimensional space. The headers of this space are: location, day, category and subtotal. Some cells define marks along an axis while others give money spent at points within this space. The markup for this table is:

<TABLE>
<CAPTION>Travel Expense Report</CAPTION>
<TR>
  <TH></TH>
  <TH>Meals</TH>
  <TH>Hotels</TH>
  <TH>Transport</TH>
  <TH>subtotal</TH>
</TR>
<TR>
  <TH id="san-jose" axis="san-jose">San Jose</TH>
</TR>
<TR>
  <TH headers="san-jose">25-Aug-97</TH>
  <TD>37.74</TD>
  <TD>112.00</TD>
  <TD>45.00</TD>
  <TD></TD>
</TR>
<TR>
  <TH headers="san-jose">26-Aug-97</TH>
  <TD>27.28</TD>
  <TD>112.00</TD>
  <TD>45.00</TD>
  <TD></TD>
</TR>
<TR>
  <TH headers="san-jose">subtotal</TH>
  <TD>65.02</TD>
  <TD>224.00</TD>
  <TD>90.00</TD>
  <TD>379.02</TD>
</TR>
<TR>
  <TH id="seattle" axis="seattle">Seattle</TH>
</TR>
<TR>
  <TH headers="seattle">27-Aug-97</TH>
  <TD>96.25</TD>
  <TD>109.00</TD>
  <TD>36.00</TD>
  <TD></TD>
</TR>
<TR>
  <TH headers="seattle">28-Aug-97</TH>
  <TD>35.00</TD>
  <TD>109.00</TD>
  <TD>36.00</TD>
  <TD></TD>
</TR>
<TR>
  <TH headers="seattle">subtotal</TH>
  <TD>131.25</TD>
  <TD>218.00</TD>
  <TD>72.00</TD>
  <TD>421.25</TD>
</TR>
<TR>
  <TH>Totals</TH>
  <TD>196.27</TD>
  <TD>442.00</TD>
  <TD>162.00</TD>
  <TD>800.27</TD>
</TR>
</TABLE>

By providing the data model in this way, authors make it possible for speech enabled-browsers to explore the table in rich ways, e.g., each cell could be spoken as a list, repeating the applicable headers before each data cell:

  San Jose, 25-Aug-97, Meals:  37.74
  San Jose, 25-Aug-97, Hotels:  112.00
  San Jose, 25-Aug-97, Transport:  45.00
 ...

The browser could also speak the headers only when they change:

San Jose, 25-Aug-97, Meals: 37.74
    Hotels: 112.00
    Transport: 45.00
  26-Aug-97, Meals: 27.28
    Hotels: 112.00
...

A.12 Sample style sheet for HTML

A.13 Emacspeak

For information, here is the list of properties implemented by Emacspeak, a speech subsystem for the Emacs editor.

(We thank T. V. Raman for the information about implementation status of aural properties.)

Value:	<number> \| <percentage> \| silent \| x-soft \| soft \| medium \| loud \| x-loud \| inherit
Initial:	medium
Applies to:	all elements
Inherited:	yes
Percentages:	refer to inherited value
Media:	aural
Computed value:	number

Value:	normal \| none \| spell-out \| inherit
Initial:	normal
Applies to:	all elements
Inherited:	yes
Percentages:	N/A
Media:	aural
Computed value:	as specified

Value:	<time> \| <percentage> \| inherit
Initial:	0
Applies to:	all elements
Inherited:	no
Percentages:	see prose
Media:	aural
Computed value:	time

Value:	<time> \| <percentage> \| inherit
Initial:	0
Applies to:	all elements
Inherited:	no
Percentages:	see prose
Media:	aural
Computed value:	time;;

Value:	[ [<time> \| <percentage>]{1,2} ] \| inherit
Initial:	see individual properties
Applies to:	all elements
Inherited:	no
Percentages:	see descriptions of 'pause-before' and 'pause-after'
Media:	aural
Computed value:	see individual properties

Value:	<uri> \| none \| inherit
Initial:	none
Applies to:	all elements
Inherited:	no
Percentages:	N/A
Media:	aural
Computed value:	absolute URI or 'none'

Value:	[ <'cue-before'> \|\| <'cue-after'> ] \| inherit
Initial:	see individual properties
Applies to:	all elements
Inherited:	no
Percentages:	N/A
Media:	aural
Computed value:	see individual properties

Value:	<uri> [ mix \|\| repeat ]? \| auto \| none \| inherit
Initial:	auto
Applies to:	all elements
Inherited:	no
Percentages:	N/A
Media:	aural
Computed value:	absolute URI, rest as specified

Value:	<angle> \| [[ left-side \| far-left \| left \| center-left \| center \| center-right \| right \| far-right \| right-side ] \|\| behind ] \| leftwards \| rightwards \| inherit
Initial:	center
Applies to:	all elements
Inherited:	yes
Percentages:	N/A
Media:	aural
Computed value:	normalized angle

Value:	<angle> \| below \| level \| above \| higher \| lower \| inherit
Initial:	level
Applies to:	all elements
Inherited:	yes
Percentages:	N/A
Media:	aural
Computed value:	normalized angle

Value:	<number> \| x-slow \| slow \| medium \| fast \| x-fast \| faster \| slower \| inherit
Initial:	medium
Applies to:	all elements
Inherited:	yes
Percentages:	N/A
Media:	aural
Computed value:	number

Value:	[[<specific-voice> \| <generic-voice> ],]* [<specific-voice> \| <generic-voice> ] \| inherit
Initial:	depends on user agent
Applies to:	all elements
Inherited:	yes
Percentages:	N/A
Media:	aural
Computed value:	as specified

Value:	<frequency> \| x-low \| low \| medium \| high \| x-high \| inherit
Initial:	medium
Applies to:	all elements
Inherited:	yes
Percentages:	N/A
Media:	aural
Computed value:	frequency

Value:	<number> \| inherit
Initial:	50
Applies to:	all elements
Inherited:	yes
Percentages:	N/A
Media:	aural
Computed value:	as specified

Value:	code \| none \| inherit
Initial:	none
Applies to:	all elements
Inherited:	yes
Percentages:	N/A
Media:	aural
Computed value:	as specified

Value:	digits \| continuous \| inherit
Initial:	continuous
Applies to:	all elements
Inherited:	yes
Percentages:	N/A
Media:	aural
Computed value:	as specified

Value:	once \| always \| inherit
Initial:	once
Applies to:	elements that have table header information
Inherited:	yes
Percentages:	N/A
Media:	aural
Computed value:	as specified

Appendix A. Aural style sheets