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Introducing HTML5: Video and Audio

Aug 23, 2010

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Multimedia accessibility

We've talked about the keyboard accessibility of the video element, but what about transcripts, captions for multimedia? After all, there is no alt attribute for video or audio as there is for <img>. The fallback content between the tags is only meant for browsers that can't cope with native video; not for people whose browsers can display the media but can't see or hear it due to disability or situation (for example, in a noisy environment or needing to conserve bandwidth).

The theory of HTML5 multimedia accessibility is excellent. The original author should make a subtitle file and put it in the container Ogg or MP4 file along with the multimedia files, and the browser will offer a user interface whereby the user can get those captions or subtitles. Even if the video is "embedded" on 1,000 different sites (simply by using an external URL as the source of the video/audio element), those sites get the subtitling information for free, so we get "write once, read everywhere" accessibility.

That's the theory. In practice, no one knows how to do this; the spec is silent, browsers do nothing. That's starting to change; at the time of this writing (May 2010), the WHATWG have added a new <track> element to the spec, which allows addition of various kinds of information such as subtitles, captions, description, chapter titles, and metadata.

The WHATWG is specifying a new timed text format called WebSRT (www.whatwg.org/specs/web-apps/current-work/multipage/video.html#websrt) for this information, which is one reason that this shadowy 29th element isn't in the W3C version of the spec. The format of the <track> element is

<track kind=captions src=captions.srt>

But what can you do right now? There is no one true approach to this problem, but here we'll present one possible (albeit hacky) interim solution.

Bruce made a proof of concept that displays individual lines of a transcript, which have been timestamped using the new HTML5 data-* attributes:

<article class=transcript lang=en>
<p><span data-begin=3 data-end=5>Hello, good evening and welcome.</span>
<span data-begin=7.35 data-end=9.25>Let's welcome Mr Last Week, singing his poptabulous hit &ldquot;If I could turn back time!&rdquot;</span>
</p>
...
</article>

JavaScript is used to hide the transcript <article>, hook into the timeupdate event of the video API, and overlay spans as plain text (therefore stylable with CSS) over (or next to) the video element, depending on the current playback time of the video and the timestamps on the individual spans. See it in action at http://dev.opera.com/articles/view/accessible-html5-video-with-javascripted-captions/. See Figure 4.6.

Figure 4.6 The script superimposes the caption over the video as delectable selectable text.

The data-* attributes (custom data attributes)

HTML5 allows custom attributes on any element. These can be used to pass information to local scripts.

Previously, to store custom data in your markup, authors would do something annoying like use classes: <input class="spaceship shields-5 lives-3 energy-75">. Then your script would need to waste time grabbing these class names, such as shields-5, splitting them at a delimiter (a hyphen in this example) to extract the value. In his book, PPK on JavaScript (New Riders, ISBN 0321423305), Peter Paul Koch explains how to do this and why he elected to use custom attributes in some HTML4 pages, making the JavaScript leaner and easier to write but also making the page technically invalid. As it's much easier to use data-shields=5 for passing name/value pairs to scripts, HTML5 legitimises and standardises this useful, real-world practice.

We're using data-begin and data-end; they could just as legitimately be data-start and data-finish, or (in a different genre of video) data-ooh-matron and data-slapandtickle. Like choosing class or id names, you should pick a name that matches the semantics.

Custom data attributes are only meant for passing information to the site's own scripts, for which there are no more appropriate attributes or elements.

The spec says "These attributes are not intended for use by software that is independent of the site that uses the attributes" and are therefore not intended to pass information to crawlers or third-party parsers. That's a job for microformats, microdata, or RDFa.

When the data-* attributes are fully supported in a browser, JavaScript can access the properties using element.dataset.foo (where the data-foo attribute contains the value). Support can be emulated using JavaScript by extending the HTMLElement object, which typically isn't possible in IE9 alpha release and below, which you can see here: http://gist.github.com/362081. Otherwise scripts can access the values via the get/setAttribute methods. The advantage of the dataset property over setAttribute is that it can be enumerated, but also, when fully implemented in browsers, setting a dataset attribute automatically sets the content attribute on the element giving you a shorthand syntax for setting custom data.

For more information, see the spec http://dev.w3.org/html5/spec/Overview.html#custom-data-attribute.

The BBC has a similar experiment at http://open.bbc.co.uk/rad/demos/html5/rdtv/episode2/ that takes in subtitles from an external JavaScript file http://open.bbc.co.uk/rad/demos/html5/rdtv/episode2/rdtv-episode2.js, which is closer to the vision of HTML5, but it doesn't have the side effect of allowing search engines to index the contents of the transcript.

Silvia Pfeiffer, a contractor for Mozilla, has some clever demos using HTML5 videos and some extra extensions (that are not part of the spec) at www.annodex.net/~silvia/itext/.

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