Accessibility/Video Accessibility

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In this page, we will collect all the information aggregated about video accessibility features for the Web. This includes:

  1. a list of video accessibility requirements
  2. a list of existing video accessibility formats
  3. a list of required and actual existing software for video accessibility
  4. proposed accessibility formats for the future.

Getting involved

The main discussions happen on the [Xiph accessibility mailing list], on the WHATWG, and the W3C HTML5 Accessibility Task Foce mainly because this is an inherently technical problem to solve. However, it needs as much breadth of input as possible, in particular from disabled people of varying grades.

The outcome of the analysis will be collected here and in various sub-pages.


Background on Ogg Theora and codecs in general.

Video Accessibility Requirements.

Caption and Subtitle Formats.

Comprison of Formats to Requirements.

Video Accessibility Resources.

Architecture Proposal.

Experiments / Drafts

Version 1 specification of itext: how to include captions and other timed text into HTML5.

Feedback on a first implementation of HTML5 captions and other time-aligned text.

Version 2 specification of itext: how to include captions and other timed text into HTML5.

Feedback on version 2 itext / itextlist specification.


Study Report 2008.

Video a11y Aug 2009.

In July 2010 several specs were added to the WHATWG HTML5 specification - this is a summary and discussion of it.

Related Pages

A first discussion on a captioning workplan.

W3C on multimedia accessibility.

Apple's list of issues around video accessibility.

Video accessibility on the WHATWG wiki.

Timed Text User Cases and Requirements.

Requirements for hypermedia annotations

Caption Review by Mark

Closed Captioning Review

Requirements for video in DAISY

Background: the issues around video accessibility

Video on the Web is a complex topic, not only from a technical, but also from a political and patent viewpoint. Even HTML5 and the WHATWG haven't figured out what baseline codec to recommend yet.

Video accessibility is an even more complex issue for similar reasons and because it is so simple to define a text format for captions, while complicated to define a text format that supports all video accessibility requirements. Diverse interests have created the plethora of existing formats, interest e.g. by old TV (IEEE, SMPTE, EBU, CEA, ATSC), online video professionals (MPEG), Web (W3C TT, SMIL), set-top boxes, games, IPTV, iTV (MHP), the Anime community, or the DVD ripping community.

The choice of textual format for video accessibility is a complex one with too many insufficient formats to choose from, e.g. QTtext, SubRip, SAMI, SMIL, DFXP, DVB, TimedText, EBU, SCC, Kate, CMML, SSA, MicroDVD, USF, SubViewer, or VOBSub (see for a more complete list. Note: this list is a combination of company names, codecs, authoring tools and other extraneous info and is not a comprehensive list of actual caption file formats used presently on TV or on-line). Also, support in frameworks is inconsistent - Adobe Flash for example support a proprietary CuePoints format ( and a simple version of the W3C TimedText format ("DFXP") ( YouTube allow submission of srt and sub file formats, but internally roll their own format.

A further complex problem is tool support. Most captioning software in use by captioning companies around the world is currently proprietary software. The few open source or freely available tools are often inappropriate.

This lack of standards around video accessibility is what the video accessibility work is about. The first focus in on how to best provide captions through Ogg. Subtitles for i18n, audio annotations for the blind, the inclusion of sign language video, transcripts, scripts, story boards, karaoke, metadata, and semantic annotations all somehow fall into the broader aim of making video generally more accessible for people of all abilities and disabilities.