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