Accurate and consistent pronunciation or presentation of spoken content by text to speech synthesis (TTS) is very important in many contexts, and critical in education, publishing, communication, entertainment, and other domains. TTS has become an important technology for providing access to digital content on the web. A variety of approaches have been used to address this need, ranging from improper use of the WAI-ARIA standard, to creation of custom and data-attributes, But there is no consistent approach and no interoperability to enable authoring of spoken presentaion guidance in HTML which can then be consumed by assistive technologies and other applications that utilize text to speech sythesis for rendering content. These efforts have not led to broad adoption of any standard by user agents, authors or assistive technologies. However, there are a variety of non-interoperable approaches that meet specific needs for some applications. It is currently a major challenge area, and many users are looking for standard-based solution.

This proposal identifies two possible technical approaches for author-controlled pronunciation of HTML content. Just as authors use CSS to style the visual presentation of web content, the Pronunciation Task Force aims to develop normative specifications for aural presentation. The Pronunciation Taks Force seeks implementors to convert the authoring techniques described into aural presentation. Feedback from implementors and authors will help the task force decide which approach to submit as the final recommendation.

We base our candidate approaches on a subset of SSML. Our selected SSML subset is carefully chosen to bring consistency and predictability to spoken presentation across a full range of assistive technologies and operating environments. Both technical approaches described in this publication carefully avoid the impass that has prevented SSML from becoming a native HTML technology and should, therefore, be generally applicable. Either approach described here satisfies our requirements for assistive technologies and are hypothesized to be useful to voice assistants which consume and present HTML content in spoken form. We seek feedback on which approach would prove most implementable across all applications of spoken presentation of web content.

For an introduction to pronunciation issue and related W3C documents, see: Pronunciation Overview.

Multi-attribute Approach for Including SSML in HTML

The multi-attribute approach uses one or more attributes with string values to add speech presentation to an HTML element. Publishers in Japan use a similar technique from EPUB 3 for the SSML phoneme element.

Edgar Allen Poe's The Raven, authored using the multi-attribute approach:

<p data-ssml-prosody-rate="slow" data-ssml-prosody-pitch="low">
    Once upon a midnight 
    <span data-ssml-phoneme-alphabet="ipa" data-ssml-phoneme-ph="ˈdrɪəri">dreary</span>
    <span data-ssml-break-time="500ms"></span>,
    while I pondered, weak
    <span data-ssml-break-time="150ms"></span> and weary,<br data-ssml-break-time="500ms" />
    Over many a quaint and curious volume of forgotten
    <span data-ssml-prosody-rate="x-slow" data-ssml-prosody-pitch="low"> lore—</span><br />
    While I nodded, nearly napping, suddenly there came a tapping,
    <br data-ssml-audio-src="/soundlibrary/wood/hits/hits_11" />
    As of some one gently rapping,
    <span data-ssml-audio-src="/soundlibrary/wood/hits/hits_11"></span>
    rapping at my chamber door.
    <span data-ssml-audio-src="/soundlibrary/wood/hits/hits_11"></span>
    <br data-ssml-audio-src="/soundlibrary/wood/hits/hits_11" />
    <span data-ssml-prosody-volume="x-soft" data-ssml-prosody-rate="medium">
      "'Tis some visitor,"
    </span>
    I muttered, <span data-ssml-prosody-volume="x-soft" data-ssml-prosody-rate="x-slow">
    <span data-ssml-phoneme-alphabet="ipa" data-ssem-phoneme-ph="tæpɪŋ">"tapping</span>
    at my chamber door—</span><br data-ssml-break-time="750ms" />
    Only this <span data-ssml-break-strength="weak"></span> and nothing
    <span data-ssml-break-strength="none"></span>
    <span data-ssml-prosody-volume="soft" data-ssml-prosody-rate="75%"> more."</span>
</p>

According the 2010 US Census, the population of <span
data-ssml-say-as='characters'>90274</span>
increased to 25209 from 24976 over the past 10 years.
					
				

Once upon a midnight <span data-ssml-alphabet="ipa" data-ssml-phoneme-ph="ˈdrɪəri">dreary</span>
					
				

<span data-ssml-sub-alias="Sodium Chloride"'>NaCL</span>
				

She said, "<span data-ssml-voice-gender="female"'>My name is Marie</span>".

Please use <span data-ssml-emphasis-level="strong"'>extreme caution.</span>
				

Take a deep breath,<span data-ssml-break-time="1s"></span> and exhale.
	
				

The tortoise, said (slowly) "<span data-ssml-prosody-rate="x-slow">
I am almost at the finish line</span>.""
				

You will hear a brief chime <span data-ssml-audio-src="/audio/chime.ogg"></span> when your time is up.
	
				

Single-attribute Approach for Including SSML in HTML

The single-attribute approach uses JSON to allow SSML compatible properties and values to be applied to textual content in HTML. This approach emerged as a means to transform content conforming to the IMS Question & Test Interoperability (QTI) Specification. The QTI standard supports inclusion of SSML in HTML for TTS tools used in educational assessment. This approach has seen preliminary implementation by TextHelp.

This approach requires authors to encode SSML functions, properties and values into JSON, for inclusion in HTML. Then TTS tools must transform the JSON attribute content into SSML. No existing W3C recommendation uses JSON strings as attribute values in HTML, but there is evidence of its use for custom applications using the data attribute. JSON has potential security concerns, and the impact of malformed JSON strings resulting from errors in authoring is among issues that need wider review. The browser, which normally attempts to address malformed HTML, can make no guarantees about the JSON strings. Implementers must decide how to handle malformed JSON.

While a JSON schema is proposed, there are potentially related standards, such as SpeakableSpecification, in development. Converting SSML to a proper JSON schema could cause confusion for implementers and authors. Often such conversions are "...not exactly 1:1 transformation, but very very close". Authoring tools are hypothesized to address these concerns, by eliminating the need for authors to directly write JSON. Techniques to facilitate authoring of speech attributes (whether multi or single-attribute approaches) are already demonstrated [ref].

Edgar Allen Poe's The Raven, authored using the single-attribute approach:

<p data-ssml='{"prosody":{"rate":"slow";"pitch":"low"}}'>
	Once upon a midnight
        <span data-ssml='{"phoneme":{"alphabet":"ipa";ph:"ˈdrɪəri"}}'>dreary</span>
	<span data-ssml="{"break":{"time":"500ms"}'></span>,
	while I pondered, weak
	<span data-ssml='{"break":{"time":"150ms"}'></span> and weary,
        <br data-ssml='{"break":{"time":"500ms"}' />
	Over many a quaint and curious volume of forgotten 
	<span data-ssml='{"prosody":{"rate":"x-slow";"pitch":"low"}}'>lore—</span><br />
	While I nodded, nearly napping, suddenly there came a tapping,
	<br data-ssml='{"audio":{"src":"/soundlibrary/wood/hits/hits_11"}}'/>
	As of some one gently rapping,
	<span data-ssml='{"audio":{"src":"/soundlibrary/wood/hits/hits_11"}}'></span>
	rapping at my chamber door.
	<span data-ssml='{"audio":{"src":"/soundlibrary/wood/hits/hits_11"}}'></span>
	<br data-ssml='{"audio":{"src":"/soundlibrary/wood/hits/hits_11"}}' />
	<span data-ssml='{"prosody":{"volume":"x-soft";"rate":"medium"}}'>
          "'Tis some visitor,"
        </span>
	I muttered, <span data-ssml='{"prosody":{"volume":"x-soft";"rate":"x-slow"}}'>
	<span data-ssml='{"phoneme":{"alphabet":"ipa";ph:"tæpɪŋ"}}'>"tapping</span>
	at my chamber door—</span><br data-ssml='{"break":{"time":"750ms"}'/>
	Only this<span data-ssml='{"break":{"strength":"weak"}'></span>
	and nothing<span data-ssml='{"break":{"strength":"none"}'> </span>
	<span data-ssml='{"prosody":{"volume":"soft";"rate":"75%"}}'>more."</span>
</p>

According the 2010 US Census, the population of <span
data-ssml='{"say-as" :
{"interpret-as":"characters"}}'>90274</span>
increased to 25209 from 24976 over the past 10 years.
						
					

Once upon a midnight <span data-ssml='{"phoneme":{"alphabet":"ipa";ph:"ˈdrɪəri"}}'>dreary</span>

<span data-ssml='{"sub":{"alias":"Sodium Chloride"}}'>NaCL</span>

She said, "<span data-ssml='{"voice":{"gender":"female"}}'>My name is Marie</span>".

Please use <span data-ssml='{"emphasis":{"level":"strong"}}'>extreme caution.</span>

Take a deep breath,<span data-ssml='{"break":{"time":"1s"}}'></span> and exhale.

The tortoise, said (slowly) "<span data-ssml='{"prosody":{"rate":"x-slow"}}'>I am almost at the finish line</span>.""

You will hear a brief chime <span data-ssml='{"audio":{"src":"/audio/chime.ogg"}}'></span> when your time is up.

Technical Considerations

In each approach, implementors must consume data-ssml- attributes, values, and associated text content to produce a valid format required by the target TTS technology. For instance, if targeting the Web Speech API, an implementation must produce valid SSML. How to determine the text content of each pronunciation function will challenge implementors of either approach.

Following the multi-attribute approach, implementors must find all related attributes and collect them into the proper SSML representation. Implementors must determine how to handle authoring errors such as missing required attributes, conflicting functions, or invalid values.

Following the single-attribute approach, implementors must parse JSON string values. Implementors must determine how to handle malformed JSON as well as incorrect property and value combinations. The use of JSON schemas could help authors and implementors of the single-attribute approach.