CSS3 Speech Module

Abstract

CSS (Cascading Style Sheets) is a language for describing the rendering of HTML and XML documents on screen, on paper, in speech, etc. CSS defines aural properties that give control over rendering XML and HTML to speech. This draft describes the text to speech properties proposed for CSS level 3, and is a re-work of the informative CSS2.1 Aural appendix [[CSS21]]. These are designed for match the model described in the Speech Synthesis Markup Language (SSML) Version 1.0 [[!SPEECH-SYNTHESIS]].

The CSS3 Speech Module is a community effort and if you would like to help with implementation and driving the specification forward along the W3C Recommendation track, please contact the editors.

Status of this document

This document is a draft of one of the "modules" for the upcoming CSS3 specification.

This document has been developed in cooperation with the Voice Browser working group (W3C Members only).

The following features are at-risk and may be dropped at the end of the CR period if there has not been enough interest from implementers: 'phoneme'

The CSS WG maintains an issues list for this module.

Table of contents

1. Dependencies on other modules

This CSS3 module depends on the following other CSS3 modules:

• Cascade and Inheritance [[CSS3CASCADE]]
• Tables [[CSS3TBL]]
• Syntax [[CSS3SYN]]
• Values and Units [[CSS3VAL]]

It has non-normative (informative) references to the following other CSS3 modules:

• The box model [[CSS3BOX]]
• Generated and Replaced Content [[CSS3GENCON]]
• Selectors [[SELECT]]

2. Introduction

The speech 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 text to speech 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 voice properties, the canvas consists of a two channel stereo space and a temporal space (you can specify audio cues before and after synthetic speech). The CSS properties also allow authors to vary the characteristics of synthetic speech (voice type, frequency, inflection, etc.).

h1, h2, h3, h4, h5, h6 {
    voice-family: paul;
    voice-stress: moderate;
    cue-before: url(ping.au)
}
p.heidi { voice-balance: left; voice-family: female }
p.peter { voice-balance: right; voice-family: male }
p.goat  { voice-volume: soft }

3. The aural "box" model

The formatting model of CSS for aural media is based on a sequence of sounds and silences that appear in a nested model which is related to the visual box model; however the aural canvas is one-dimensional, monolinear. For compatibility with the visual box model, we will call it the aural "box" model.

The element is surrounded by, in this order, 'rest', 'cue' and 'pause' properties - they can be thought of as aural equivalents to 'padding', 'border' and 'margin' respectively.

It can be represented in the following way (including the equivalent properties from the visual box model for clarification of relationships):

where element is the selected element to which the properties from the Speech Module apply.

4. Mixing properties: 'voice-volume' and 'voice-balance'

The 'voice-volume' refers to the amplitude of the waveform output by the speech synthesiser. This may be mixed with other audio sources, influencing the perceived loudness of synthetic speech relative to these sources. Note that voice-volume does not apply to audio cues for which there is a separate means to set the relative loudness.

Values have the following meanings:

<number>

An integer or floating point number in the range '0' to '100'. '0' represents silence (the minimum level), and 100 corresponds to the maximum level. The volume scale is linear amplitude.

<percentage>

Percentage values are calculated relative to the inherited value, and are then clipped to the range '0' to '100'.

silent, x-soft, soft, medium, loud, and x-loud

A sequence of monotonically non-decreasing volume levels. The value of 'silent' is mapped to '0' and 'x-loud' is mapped to '100'. The mapping of other values to numerical volume levels is implementation dependent and may vary from one speech synthesizer to the next.

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

The 'voice-balance' property refers to the balance between left and right channels, and presumes a two channel (stereo) model that is widely supported on consumer audio equipment.

Values have the following meanings:

<number>

An integer or floating point number between '-100' and '100'. For '-100' only the left channel is audible. Simarly for '100' or '+100' only the right channel is audible. For '0' both channels have the same level, so that the speech appears to be coming from the center.

left

Same as '-100'.

center

Same as '0'.

right

Same as '100' or '+100'.

leftwards

Moves the sound to the left, relative to the inherited voice balance. More precisely, subtract 20 from the inherited value and clip the resulting value to the range '-100' and '100'.

rightwards

Moves the sound to the right, relative to the inherited voice balance. More precisely, add 20 to the inherited value and clip the resulting value to the range '-100' and '100'.

Many speech synthesizers only support a single channel. The voice-balance property can then be treated as part of a post synthesis mixing step. This is where speech is mixed with other audio sources. Note that unlike voice-volume, voice-balance does apply to cues.

5. Speaking properties: 'speak'

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

none

Suppresses aural rendering so that the element requires no time to render. Note, however, that descendants may override this value and will be spoken. (To be sure to suppress rendering of an element and its descendants, use the 'display' property [[CSS3BOX]]).

normal

Uses language-dependent pronunciation rules for rendering an element and its children. Punctuation is not to be spoken, but instead rendered naturally as various pauses.

spell-out

Spells the text one letter at a time (useful for acronyms and abbreviations). In languages where accented characters are rare, it is permitted to drop accents in favor of alternative unaccented spellings. As as example, in English, the word "rôle" can also be written as "role". A conforming implementation would thus be able to spell-out "rôle" as "R O L E".

digits

Speak numbers one digit at a time, for instance, "twelve" would be spoken as "one two", and "31" as "three one".

literal-punctuation

Similar to 'normal' value, but punctuation such as semicolons, braces, and so on are to be spoken literally.

no-punctuation

Similar to 'normal' value but punctuation is not to be spoken nor rendered as various pauses.

Note the difference between an element whose 'voice-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).

Speech synthesizers are knowledgeable about what is and what is not a number. The speak property gives authors the means to control how the synthesizer renders the numbers it discovers in the source text, and may be implemented as a preprocessing step before passing the text to the speech synthesizer.

6. Pause properties: 'pause-before', 'pause-after' and 'pause', collapsing pauses

These properties specify a pause or prosodic boundary to be observed before (or after) an element or, if any 'cue-before' or 'cue-after' is specified, before (or after) these. Values have the following meanings:

<time>

Expresses the pause in absolute time units (seconds and milliseconds, e.g. "3s", "250ms"). Only positive values are allowed.

none, x-weak, weak, medium, strong, and x-strong

These values may be used to indicate the prosodic strength of the break in speech output. The synthesis processor may insert a pause as part of its implementation of the prosodic break. The value "none" indicates that no prosodic break boundary should be output, and can be used to inhibit a prosodic break which the processor would otherwise produce. The other values indicate monotonically non-decreasing (conceptually increasing) break strength between elements. The stronger boundaries are typically accompanied by pauses. "x-weak" and "x-strong" are mnemonics for "extra weak" and "extra strong", respectively. The breaks between paragraphs are typically much stronger than the breaks between words within a sentence.

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

h1 { pause: 20ms } /* pause-before: 20ms; pause-after: 20ms */
h2 { pause: 30ms 40ms } /* pause-before: 30ms; pause-after: 40ms */
h3 { pause-after: 10ms } /* pause-before: unspecified; pause-after: 10ms */

Collapsing pauses

The pause defines the minimum distance of the aural "box" to the aural "boxes" before and after it. Adjacent pauses should be merged by selecting the strongest named break or the longest absolute time interval. Thus "strong" is selected when comparing "strong" and "weak", while "1s" is selected when comparing "1s" and "250ms". We say that such pauses collapse. A combination of a named break and time duration is treated additively.

The following pauses collapse:

1. 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.
2. 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.
3. The pause-after of an aural "box" and the pause-before of its next sibling.
4. The pause-before and pause-after of an aural "box", if the the "box" has a voice-duration of "0ms", no rest-before or rest-after, no cue-before or cue-after and no content.

The pause-after of an element is always adjoining to the pause-before of its next sibling.

The pause-before an element is adjoining to its first child's pause-before, if the element has no cue-before nor rest-before.

The pause-after of an element is adjoining to its last child's pause-after, if the element has no rest-after or cue-after.

An element's own pauses are adjoining if the 'voice-duration' property is zero, and it has no rest, and it does not contain any content, and all of its children's pauses (if any) are adjoining.

Editor's note: 'pause' has been re-defined, and 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 [[CSS21]].

7. Rest properties: rest-before', 'rest-after' and 'rest'

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

<time>

Expresses the rest in absolute time units (seconds and milliseconds, e.g. "3s", "250ms"). Only positive values are allowed.

none, x-weak, weak, medium, strong, and x-strong

These values may be used to indicate the prosodic strength of the break in speech output. The synthesis processor may insert a rest as part of its implementation of the prosodic break. The value "none" indicates that no prosodic break boundary should be output, and can be used to inhibit a prosodic break which the processor would otherwise produce. The other values indicate monotonically non-decreasing (conceptually increasing) break strength between words. The stronger boundaries are typically accompanied by rests. "x-weak" and "x-strong" are mnemonics for "extra weak" and "extra strong", respectively.

As opposed to pause properties, the rest is inserted between the element's content and any 'cue-before' or 'cue-after' content. Adjacent rests are treated additively.

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

8. 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:

<uri>

The URI must designate an auditory icon resource. If the URI resolves to something other than an audio file, such as an image, the resource should be ignored and the property treated as if it had the value 'none'.

none

No auditory icon is specified.

<number>

An integer or floating point number in the range '0' to '100'. '0' represents silence (the minimum level), and 100 corresponds to the maximum level. The volume scale is linear amplitude.

<percentage>

Percentage values are calculated relative to the inherited value of the 'voice-volume' property, and are then clipped to the range '0' to '100'. By basing the percentage upon the inherited value for voice-volume, it is easy to adjust the relative loudness of cues compared to synthetic speech for whatever volume setting has been provided for that speech.

silent, x-soft, soft, medium, loud, and x-loud

A sequence of monotonically non-decreasing volume levels. The value of 'silent' is mapped to '0' and 'x-loud' is mapped to '100'. The mapping of other values to numerical volume levels is implementation dependent, but the intention is to match the corresponding levels for voice-volume.

a { cue-before: url(bell.aiff); cue-after: url(dong.wav) }

h1 { cue-before: url(pop.au) 80; cue-after: url(pop.au) 50% }

div.caution { cue-before: url(caution.wav) loud }

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.

h1 {cue-before: url(pop.au); cue-after: url(pop.au) }

h1 {cue: url(pop.au) }

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 (e.g., popping up a warning, emitting a warning sound, etc.)

Please see the sections on the ::before and ::after pseudo-elements [[SELECT]] for information on other content generation techniques.

9. Mark properties: 'mark-before', 'mark-after' and 'mark'

The mark properties allow named markers to be attached to the audio stream. For compatibility with SSML, this must conform to the xsd:token datatype as defined in XML Schema. Synthesis processors must do one or both of the following when encountering a mark:

• Inform the hosting environment with the value of the mark's name with information allowing the platform to retrieve the corresponding position in the rendered output.
• When audio output of the SSML document reaches the mark, issue an event that includes the required name attribute of the element. The hosting environment defines the destination of the event.

The mark properties have no audible effect on the speech and instead just serve to mark points in the stream.

Values have the following meanings:

<string>

A string to be used as the name of the mark.

A good way of adding the marking properties could be by means of using the attr() function.

h1 {mark-before: "section"}

p {mark-before: attr(id) }

The 'mark' property is a shorthand for setting 'mark-before' and 'mark-after'. If two values are given the first value is 'mark-before' and the second is 'mark-after'. If only one value is given, it applies to both properties.

div {mark-before: "start"; mark-after: "end" }

div {mark: "start" "end" }

10. Voice characteristic properties: 'voice-family', 'voice-rate', 'voice-pitch', 'voice-pitch-range' and 'voice-stress'

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

<specific-voice>

Values are specific instances (e.g., comedian, mary, carlos, "valley-girl").

<age>

Possible values are 'child', 'young' and 'old'.

<generic-voice>

Values are voice families. Possible values are 'male', 'female' and 'neutral'.

<number>

Indicates a preferred variant of the other voice characteristics (e.g. the second or next male voice). Possible values are positive integers. '1' is the first of all available voices and is equal to no numeric value.

h1 { voice-family: announcer, old male }

p.part.romeo  { voice-family: romeo, young male }

p.part.juliet { voice-family: juliet, female }

p.part.mercutio { voice-family: male 2 }

p.part.tybalt { voice-family: male 3 }

p.part.nurse { voice-family: child female }

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 [[CSS3SYN]]. Any whitespace characters before and after the voice name are ignored. For compatibility with SSML, whitespace characters are not permitted within voice names.

The voice-family property is used to guide the selection of the voice to be used for speech synthesis. The overriding priority is to match the language specified by the xml:lang attribute as per the XML 1.0 specification [[XML10]], and as inherited by nested elements until overridden by a further xml:lang attribute.

If there is no voice available for the requested value of xml:lang, the processor should select a voice that is closest to the requested language (e.g. a variant or dialect of the same language). If there are multiple such voices available, the processor should use a voice that best matches the values provided with the voice-volume property. It is an error if there are no such matches.

This property controls the speaking rate. The default rate for a voice depends on the language and dialect and on the personality of the voice. The default rate for a voice should be such that it is experienced as a normal speaking rate for the voice when reading aloud text. Since voices are processor-specific, the default rate will be as well.

<percentage>

Applies to the default speaking rate for each voice. Thus 50% means half the normal rate for this voice.

x-slow, slow, medium, fast and x-fast

A sequence of monotonically non-decreasing speaking rates that are implementation and voice specific.

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.

Values have the following meanings:

<number>

A positive integer or floating point number that specifies the average pitch of the speaking voice in Hertz.

<percentage>

Specifies a relative change to the inherited value.

x-low, low, medium, high, x-high

A sequence of monotonically non-decreasing pitch levels that are implementation and voice specific.

ISSUE: should we also allow for relative changes in terms of semitones as permitted by SSML?

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.

Values have the following meanings:

<number>

An non-negative integer or floating point number indicating pitch range in Hertz. Low ranges produce a flat, monotonic voice. A high range produces animated voices.

<percentage>

Specifies a relative change to the inherited value.

x-low, low, medium, high and x-high

A sequence of monotonically non-decreasing pitch ranges that are implementation and language dependent.

ISSUE: should we also allow for relative changes in terms of semitones as permitted by SSML?

Indicates the strength of emphasis to be applied. Emphasis is indicated using a combination of pitch change, timing changes, loudness and other acoustic differences) that varies from one language to the next.

Values have the following meanings:

none, moderate and strong

These are monotonically non-decreasing in strength, with the precise meanings dependent on language being spoken. The value 'none' inhibits the synthesizer from emphasizing words it would normally emphasize.

reduced

Effectively the opposite of emphasizing a word. For example, when the phrase "going to" is reduced it may be spoken as "gonna".

11. Duration property: 'voice-duration'

Allows authors to specify how long it should take to render the selected element's content. This property overrides the 'voice-rate' property. Values have the following meanings:

<time>

Specifies a value in seconds or milliseconds for the desired time to take to speak the element contents, for instance, "250ms", or "3s". Only positive numbers are allowed.

12. Phonetics: 'phonemes', '@phonetic-alphabet' and 'content'

This allows authors to specify a phonetic pronunciation for the text contained by the corresponding element.

@phonetic-alphabet

The default alphabet for the pronunciation string of the 'phonemes' property is the International Phonetic Alphabet ("ipa"), corresponding to Unicode representations of the phonetic characters developed by the International Phonetic Association [IPA]. The phonetic alphabet can be explicitly specified using the @phonetic-alphabet rule, for instance:

@phonetic-alphabet "ipa";
#tomato { phonemes: "t\0252 m\0251 to\028a " }

At most one @phonetic-alphabet rule may appear in style sheet and it must appear, when used, before any occurrence of the 'phonemes' property.

ISSUE: we do not mention any other possible value for @phonetic-alphabet. SSML 1.0 defines "ipa" to be the only valid value - any other value should be added with the "x-" prefix. Do we want to follow this? Or do we allow some other values? SAMPA comes to my mind as a widely used ASCII alternative to IPA, but then: SAMPA is language dependent, and it might be better to have X-SAMPA.

Inserted and replaced content

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. In the following example, the abbreviation is rendered using the content of the title attribute instead of the element's content:

abbr { content: attr(title); }
  ...

<abbr title="World Wide Web Consortium">W3C</abbr>

In a similar way text-to-speech strings in a document can be replaced by a previously recorded version:

.hamlet { content: url(gielgud.wav); }
  ...

<div class="hamlet">
To be, or not to be: that is the question:
</div>

Furthermore authors, or users in a user stylesheet, may want to add some information to ease understanding the structure for non-visual interaction with the document. They can do so by using the '::before' and '::after' pseudo-elements that will be inserted between the element's contents and the 'rest':

ul::before { content: "Start list: " }
ul::after  { content: "List end. " }
li::before { content: "List item: " }

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

Editor's note: The alphabet is specified via an at-rule to avoid problems with inappropriate cascades that can occur if the alphabet was set via a property.

   say-instead: "as-key";

Appendix A — Changes from previous draft

• Fixed "voice-family" grammar as per this discussion
• Added "HTML" to "CSS defines aural properties that give control over rendering XML to speech" in the abstract.
• Reworked HTML source code to work with the members-only W3C pre-processor/generator
• Fixed minor typos

Appendix B — Profiles

TBD

Appendix C — Acknowledgements

The editors would like to thank the members of the W3C Voice Browser and Cascading Style Sheets working groups for their assistance in preparing this new draft. Special thanks to Ellen Eide (IBM) for her detailed comments.

Appendix D — Property index

The following properties are defined in other modules:

• content [[CSS3GENCON]]
• display [[CSS3BOX]]

Appendix E — Index

Appendix F — Definitions

Glossary

The following terms and abbreviations are used in this module.

UA

User Agent

A program that reads and/or writes CSS style sheets on behalf of a user in either or both of these categories: programs whose purpose is to render documents (e.g., browsers) and programs whose purpose is to create style sheets (e.g., editors). A UA may fall into both categories. (There are other programs that read or write style sheets, but this module gives no rules for them.)

document

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

style sheet

A CSS style sheet.

Conformance

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification. All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [[!RFC2119]]

Examples in this specification are introduced with the words "for example" or are set apart from the normative text with class="example", like this:

Informative notes begin with the word "Note" and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Conformance to CSS Backgrounds and Borders Level 3 is defined for three classes:

style sheet

A CSS style sheet.

renderer

A UA that interprets the semantics of a style sheet and renders documents that use them.

authoring tool

A UA that writes a style sheet.

A style sheet is conformant to CSS Backgrounds and Borders Level 3 if all of its declarations that use properties defined in this module have values that are valid according to the generic CSS grammar and the individual grammars of each property as given in this module.

A renderer is conformant to CSS Backgrounds and Borders Level 3 if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the properties defined by CSS Backgrounds and Borders Level 3 by parsing them correctly and rendering the document accordingly. However the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to CSS Backgrounds and Borders Level 3 if it writes syntactically correct style sheets, according to the generic CSS grammar and the individual grammars of each property in this module.

Levels

This section is informative. 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.

CSS Level 1

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

CSS Level 2

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

CSS Level 3

• All features described in the CSS Backgrounds and Borders Module Level 3

CR exit criteria

As described in the W3C process document, a Candidate Recommendation (CR) is a specification that W3C recommends for use on the Web. The next stage is "Recommendation" when the specification is sufficiently implemented.

For this specification to be proposed as a W3C Recommendation, the following conditions shall be met. There must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:

independent

each implementation must be developed by a different party and cannot share, reuse, or derive from code used by another qualifying implementation. Sections of code that have no bearing on the implementation of this specification are exempt from this requirement.

interoperable

passing the respective test case(s) in the official CSS test suite, or, if the implementation is not a Web browser, an equivalent test. Every relevant test in the test suite should have an equivalent test created if such a user agent (UA) is to be used to claim interoperability. In addition if such a UA is to be used to claim interoperability, then there must one or more additional UAs which can also pass those equivalent tests in the same way for the purpose of interoperability. The equivalent tests must be made publicly available for the purposes of peer review.

implementation

a user agent which:

1. implements the specification.
2. is available to the general public. The implementation may be a shipping product or other publicly available version (i.e., beta version, preview release, or "nightly build"). Non-shipping product releases must have implemented the feature(s) for a period of at least one month in order to demonstrate stability.
3. is not experimental (i.e., a version specifically designed to pass the test suite and is not intended for normal usage going forward).

A minimum of sixth months of the CR period must have elapsed. This is to ensure that enough time is given for any remaining major errors to be caught.

Features will be dropped if two or more interoperable implementations are not found by the end of the CR period.

Features may/will also be dropped if adequate/sufficient (by judgment of CSS WG) tests have not been produced for those feature(s) by the end of the CR period.

Appendix G — References

Normative references

[IPA]

International Phonetic Association

Other references