Multimedia accessibility

We’ve talked about the keyboard accessibility of the video element, but what about transcripts and 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 meant only 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, being in a noisy environment or needing to conserve bandwidth).

There are two methods of attaching synchronized text alternatives (captions, subtitles, and so on) to multimedia, called in-band and out-of-band. In-band means that the text file is included in the multimedia container; an MP4 file, for example, is actually a container for H.264 video and AAC audio, and can hold other metadata files too, such as subtitles. WebM is a container (based on the open standard Matroska Media Container format) that holds VP8 video and Ogg Vorbis audio. Currently, WebM doesn’t support subtitles, as Google is waiting for the Working Groups to specify the HTML5 format: “WHATWG/W3C RFC will release guidance on subtitles and other overlays in HTML5 <video> in the near future. WebM intends to follow that guidance”. (Of course, even if the container can contain additional metadata, it’s still up to the media player or browser to expose that information to the user.)