When we talk about Web video here, we explicitly refer to video published in Ogg Theora/Vorbis format inside a Web browser that supports the HTML5 video element.
Accessibility of video refers to several different aspects of usability of video, depending on what user group we are looking at. So, before diving into the different aspects and how they can be supported, we list the user groups and their specific requirements.
A key accessibility challenge for browser vendors with the HTML5 video element is to make the default controls accessible through the keyboard. The HTML5 video element provides an attribute called controls which requests the browser to create default controls on top of the video.
Here is what the current specification says:
“This user interface should include features to begin playback, pause playback, seek to an arbitrary position in the content (if the content supports arbitrary seeking), change the volume, and show the media content in manners more suitable to the user (e.g. full-screen video or in an independent resizable window).”
In Firefox 3.5, the controls attribute currently creates the following controls:
Further, the HTML5 specification prescribes that if the controls attribute is not available, “user agents may provide controls to affect playback of the media resource (e.g. play, pause, seeking, and volume controls), but such features should not interfere with the page’s normal rendering. For example, such features could be exposed in the media element’s context menu.”
In Firefox 3.5, this has been implemented with a right-click context menu, which contains:
When the controls are being displayed, there are keyboard shortcuts to control them:
To make these controls accessible to VI users, Firefox exposes them to screen readers using MSAA or AT-SPI. It implies having to use focus mode for now. Exposure through iSimpleDOM interfaces on Windows (http://www.marcozehe.de/2009/06/11/exposure-of-audio-and-video-elements-to-assistive-technologies/) are still in development. Once in focus mode, the keyboard shortcuts listed above make the video controls accessible.
As described above, accessibility for a particular video is provided through creating additional data that accompanies the original video. A fully accessible video may consist of all of the following:
All of the mentioned data that provides accessibility to video is time-aligned with the original video. It can be provided in two different ways:
Several demos have been made with out-of-band subtitles, captions, and audio descriptions and the HTML5 video tag:
Silvia Pfeiffer's demo includes a proposal for how to associate out-of-band accessibility data through a new HTML5 tag with videos. This specification is continuing to evolve and is expected to eventually lead to native browser support of time-aligned accessibility data. A similar proposal has been made by Greg Millam from Google (http://lists.whatwg.org/pipermail/whatwg-whatwg.org/2009-February/018600.html). Such a specification may look as follows:
<video src="../elephant.ogv" poster="elephant.png" controls> <itext lang="fr" type="text/srt" src="../elephant.fr.srt" category="SUB"></itext> <itext lang="en" type="text/srt" src="../elephant.en.srt" category="CC"></itext> <itext lang="en" type="text/srt" src="../audiodesc.srt" category="TAD"></itext> </video>
Note: this is an example proposal only, which is not currently supported natively by any browsers.