Tag Archives: accessibility

Introducing media accessibility into HTML5

In recent months, people in the W3C HTML5 Accessibility Task Force developed two proposals for introducing caption, subtitle, and more generally time-aligned text support into HTML5 audio and video.

These time-aligned text files can either come as external files that are associated with the timeline of the media resource, or they come as part of the media resource in a binary track.

For both cases we now have proposals to extend the HTML5 specification.

Firstly, let’s look at time-aligned text in external files. The change proposal introduces markup to associate such external files as a kind of “virtual track” with a media resource. Here is an example:

<video src="video.ogv">
<track src="video_cc.ttml" type="application/ttaf+xml" language="en" role="caption"></track>
<track src="video_tad.srt" type="text/srt" language="en" role="textaudesc"></track>
<trackgroup role="subtitle">
<track src="video_sub_en.srt" type="text/srt; charset='Windows-1252'" language="en"></track>
<track src="video_sub_de.srt" type="text/srt; charset='ISO-8859-1'" language="de"></track>
<track src="video_sub_ja.srt" type="text/srt; charset='EUC-JP'" language="ja"></track>

The video resource is “video.ogv”. Associated with it are five timed text resources.

The first one is written in TTML (which is the new name for DFXP), is a caption track and in English. TTML is particularly useful when you want to provide more than just an unformatted piece of text to the viewers. Hearing-impaired users appreciate any visual help they can be provided with to absorb the caption text more quickly. This includes colour coding of speakers, positioning of text close to the speaking person on screen, or even animated musical notes to signify music. Thus, a format like TTML that allows for formatting and positioning information is an appropriate format to specify captions.

All other timed text resources are provided in SRT format, which is a simpler format that TTML with only plain text in the text cues.

The second text track is a textual audio description track. A textual audio description is in fact targeted at the vision-impaired and contains text that is expected to be read out by a screen reader or routed to a braille device. Thus, as the video plays, a vision-impaired user receives additional information about the visual content of the scene through their screen reader or braille device. The SRT format is particularly useful for providing textual audio descriptions since it only provides plain text, which can easily be handed on to assistive technology. When authoring such textual audio descriptions, it is very important to pick time intervals in the original media resource where no other significant audio cue is provided, such that the vision-impaired user is able to listen to the screen reader during that time.

The last three text tracks are subtitle tracks. They are grouped into a trackgroup element, which is not strictly necessary, but enables the author to say that these tracks are supposed to be alternatives. Thus, a Web Browser can create a menu with all the available tracks and put the tracks in the trackgroup into a menu of their own where only one option is selectable (similar to how radiobuttons work). Incidentally, the trackgroup element also allows to avoid having to repeat the role attribute in all the containing tracks. It is expected that these menus will be added to the default media controls and will thus be visible if the media element has a controls attribute.

With the role, type and language attributes, it is easy for a Web Browser to understand what the different tracks have to offer. A Web Browser can even decide to offer new functionality that is helpful to certain user groups. For example, an addition to a Web Browser’s default settings could be to allow users to instruct a Web Browser to always turn on captions or subtitles if they are available in the user’s main language. Or to always turn on textual audio descriptions. In this way, a user can customise their default experience of a media resource over and on top of what a Web page author decides to expose.

Incidentally, the choice of “track” as a name for relating external text resources to a media element has a deeper meaning. It is easily possible in future to extend “track” elements to not just point to dependent text resources, but also to dependent audio or video resources. For example, an actual audio description that is a recording of a human voice rather than a rendered text description could be association in the same way. Right now, such an implementation is not envisaged by the Browser vendors, but it will be something to work towards in the future.

Now, with such functionality available, there is naturally a desire to be able to control activation or de-activation of text tracks through JavaScript, not just through user interaction. A Web Developer may for example want to override the default controls provided by a Web Browser and run their own JavaScript-based controls, thus requiring to create their own selection menu for the tracks.

This is actually also an issue more generally and applies to all track types, including such tracks that come inside an existing media resource. In the current specification such tracks are not exposed and can therefore not be activated.

This is where the second specification that the W3C Accessibility Task Force has worked towards comes in: the media multitrack JavaScript API.

This specification introduces a read-only JavaScript interface to the audio and video elements to allow Web Developers to find out about the tracks (including the virtual tracks) that a media resource offers. The only action that the interface currently provides is to enable or disable tracks.
Here is an example use to turn on a french subtitle track:

if (video.tracks[2].role == "subtitle" && video.tracks[2].language == "fr") video.tracks[2].enabled = true;

There is still a need to introduce a means to actually expose the text cues as they relate to the currentTime of the media resource. This has not yet been specified in the given proposals.

The text cues could be exposed in several ways. They could be exposed through introducing an event, i.e. every time a new text cue becomes active, a callback is called which is given the active text cue (if such a callback had been registered previously). Another option is to simply write the text cues into a specified div-element in the DOM and thus expose them directly in the Browser. A third idea could be to expose the text cues in an iframe-like element to avoid any cross-site security issues. And a fourth idea that we have discussed is to expose the text cues in an attribute of the track.

All of this obviously also relates to how to actually render the text cues and whether to render them in a shadow DOM so as to make the JavaScript reading separate from the rendering and address security and copyright issues. I’d be curious in opinions here on how it should be done.

HTML5 Media and Accessibility presentation

Today, I was invited to give a talk at my old workplace CSIRO about the HTML5 media elements and accessibility.

A lot of the things that have gone into Ogg and that are now being worked on in the W3C in different working groups – including the Media Fragments and HTML5 WGs – were also of concern in the Annodex project that I worked on while at CSIRO. So I was rather excited to be able to report back about the current status in HTML5 and where we’re at with accessibility features.

Check out the presentation here. It contains a good collection of links to exciting demos of what is possible with the new HTML5 media elements when combined with other HTML features.

I tried something now with this presentation: I wrote it in a tool called S5, which makes use only of HTML features for the presentation. It was quite a bit slower than I expected, e.g. reloading a page always included having to navigate to that page. Also, it’s not easily possible to do drawings, unless you are willing to code them all up in HTML. But otherwise I have found it very useful for, in particular, including all the used URLs and video element demos directly in the slides. I was inspired with using this tool by Chris Double’s slides from LCA about implementing HTML 5 video in Firefox.

Accessibility support in Ogg and liboggplay

At the recent FOMS/LCA in Wellington, New Zealand, we talked a lot about how Ogg could support accessibility. Technically, this means support for multiple text tracks (subtitles/captions), multiple audio tracks (audio descriptions parallel to main audio track), and multiple video tracks (sign language video parallel to main video track).

Creating multitrack Ogg files
The creation of multitrack Ogg files is already possible using one of the muxing applications, e.g. oggz-merge. For example, I have my own little collection of multitrack Ogg files at http://annodex.net/~silvia/itext/elephants_dream/multitrack/. But then you are stranded with files that no player will play back.

Multitrack Ogg in Players
As Ogg is now being used in multiple Web browsers in the new HTML5 media formats, there are in particular requirements for accessibility support for the hard-of-hearing and vision-impaired. Either multitrack Ogg needs to become more of a common case, or the association of external media files that provide synchronised accessibility data (captions, audio descriptions, sign language) to the main media file needs to become a standard in HTML5.

As it turn out, both these approaches are being considered and worked on in the W3C. Accessibility data that are audio or video tracks will in the near future have to come out of the media resource itself, but captions and other text tracks will also be available from external associated elements.

The availability of internal accessibility tracks in Ogg is a new use case – something Ogg has been ready to do, but has not gone into common usage. MPEG files on the other hand have for a long time been used with internal accessibility tracks and thus frameworks and players are in place to decode such tracks and do something sensible with them. This is not so much the case for Ogg.

For example, a current VLC build installed on Windows will display captions, because Ogg Kate support is activated. A current VLC build on any other platform, however, has Ogg Kate support deactivated in the build, so captions won’t display. This will hopefully change soon, but we have to look also beyond players and into media frameworks – in particular those that are being used by the browser vendors to provide Ogg support.

Multitrack Ogg in Browsers
Hopefully gstreamer (which is what Opera uses for Ogg support) and ffmpeg (which is what Chrome uses for Ogg support) will expose all available tracks to the browser so they can expose them to the user for turning on and off. Incidentally, a multitrack media JavaScript API is in development in the W3C HTML5 Accessibility Task Force for allowing such control.

The current version of Firefox uses liboggplay for Ogg support, but liboggplay’s multitrack support has been sketchy this far. So, Viktor Gal – the liboggplay maintainer – and I sat down at FOMS/LCA to discuss this and Viktor developed some patches to make the demo player in the liboggplay package, the glut-player, support the accessibility use cases.

I applied Viktor’s patch to my local copy of liboggplay and I am very excited to show you the screencast of glut-player playing back a video file with an audio description track and an English caption track all in sync:


Further developments
There are still important questions open: for example, how will a player know that an audio description track is to be played together with the main audio track, but a dub track (e.g. a German dub for an English video) is to be played as an alternative. Such metadata for the tracks is something that Ogg is still missing, but that Ogg can be extended with fairly easily through the use of the Skeleton track. It is something the Xiph community is now working on.

This is great progress towards accessibility support in Ogg and therefore in Web browsers. And there is more to come soon.

Audio Track Accessibility for HTML5

I have talked a lot about synchronising multiple tracks of audio and video content recently. The reason was mainly that I foresee a need for more than two parallel audio and video tracks, such as audio descriptions for the vision-impaired or dub tracks for internationalisation, as well as sign language tracks for the hard-of-hearing.

It is almost impossible to introduce a good scheme to deliver the right video composition to a target audience. Common people will prefer bare a/v, vision-impaired would probably prefer only audio plus audio descriptions (but will probably take the video), and the hard-of-hearing will prefer video plus captions and possibly a sign language track . While it is possible to dynamically create files that contain such tracks on a server and then deliver the right composition, implementation of such a server method has not been very successful in the last years and it would likely take many years to roll out such new infrastructure.

So, the only other option we have is to synchronise completely separate media resource together as they are selected by the audience.

It is this need that this HTML5 accessibility demo is about: Check out the demo of multiple media resource synchronisation.

I created a Ogg video with only a video track (10m53s750). Then I created an audio track that is the original English audio track (10m53s696). Then I used a Spanish dub track that I found through BlenderNation as an alternative audio track (10m58s337). Lastly, I created an audio description track in the original language (10m53s706). This creates a video track with three optional audio tracks.

I took away all native controls from these elements when using the HTML5 audio and video tag and ran my own stop/play and seeking approaches, which handled all media elements in one go.

I was mostly interested in the quality of this experience. Would the different media files stay mostly in sync? They are normally decoded in different threads, so how big would the drift be?

The resulting page is the basis for such experiments with synchronisation.

The page prints the current playback position in all of the media files at a constant interval of 500ms. Note that when you pause and then play again, I am re-synching the audio tracks with the video track, but not when you just let the files play through.

I have let the files play through on my rather busy Macbook and have achieved the following interesting drift over the course of about 9 minutes:

Drift between multiple parallel played media elements

You will see that the video was the slowest, only doing roughly 540s, while the Spanish dub did 560s in the same time.

To fix such drifts, you can always include regular re-synchronisation points into the video playback. For example, you could set a timeout on the playback to re-sync every 500ms. Within such a short time, it is almost impossible to notice a drift. Don’t re-load the video, because it will lead to visual artifacts. But do use the video’s currentTime to re-set the others. (UPDATE: Actually, it depends on your situation, which track is the best choice as the main timeline. See also comments below.)

It is a workable way of associating random numbers of media tracks with videos, in particular in situations where the creation of merged files cannot easily be included in a workflow.

Manifests for exposing the structure of a Composite Media Resource

In the previous post I explained that there is a need to expose the tracks of a time-linear media resource to the user agent (UA). Here, I want to look in more detail at different possibilities of how to do so, their advantages and disadvantages.

Note: A lot of this has come out of discussions I had at the recent W3C TPAC and is still in flux, so I am writing this to start discussions and brainstorm.

Declarative Syntax vs JavaScript API

We can expose a media resource’s tracks either through a JavaScript function that can loop through the tracks and provide access to the tracks and their features, or we can do this through declarative syntax.

Using declarative syntax has the advantage of being available even if JavaScript is disabled in a UA. The markup can be parsed easily and default displays can be prepared without having to actually decode the media file(s).

OTOH, it has the disadvantage that it may not necessarily represent what is actually in the binary resource, but instead what the Web developer assumed was in the resource (or what he forgot to update). This may lead to a situation where a “404” may need to be given on a media track.

A further disadvantage is that when somebody copies the media element onto another Web page, together with all the track descriptions, and then the original media resource is changed (e.g. a subtitle track is added), this has not the desired effect, since the change does not propagate to the other Web page.

For these reasons, I thought that a JavaScript interface was preferable over declarative syntax.

However, recent discussions, in particular with some accessibility experts, have convinced me that declarative syntax is preferable, because it allows the creation of a menu for turning tracks on/off without having to even load the media file. Further, declarative syntax allows to treat multiple files and “native tracks” of a virtual media resource in an identical manner.

Extending Existing Declarative Syntax

The HTML5 media elements already have declarative syntax to specify multiple source media files for media elements. The <source> element is typically used to list video in mpeg4 and ogg format for support in different browsers, but has also been envisaged for different screensize and bandwidth encodings.

The <source> elements are generally meant to list different resources that contribute towards the media element. In that respect, let’s try using it for declaring a manifest of tracks of the virtual media resource on an example:

    <source id='av1' src='video.3gp' type='video/mp4' media='mobile' lang='en'
                     role='media' >
    <source id='av2' src='video.mp4' type='video/mp4' media='desktop' lang='en'
                     role='media' >
    <source id='av3' src='video.ogv' type='video/ogg' media='desktop' lang='en'
                     role='media' >
    <source id='dub1' src='video.ogv?track=audio[de]' type='audio/ogg' lang='de'
                     role='dub' >
    <source id='dub2' src='audio_ja.oga' type='audio/ogg' lang='ja'
                     role='dub' >
    <source id='ad1' src='video.ogv?track=auddesc[en]' type='audio/ogg' lang='en'
                     role='auddesc' >
    <source id='ad2' src='audiodesc_de.oga' type='audio/ogg' lang='de'
                     role='auddesc' >
    <source id='cc1' src='video.mp4?track=caption[en]' type='application/ttaf+xml'
                     lang='en' role='caption' >
    <source id='cc2' src='video.ogv?track=caption[de]' type='text/srt; charset="ISO-8859-1"'
                     lang='de' role='caption' >
    <source id='cc3' src='caption_ja.ttaf' type='application/ttaf+xml' lang='ja'
                     role='caption' >
    <source id='sign1' src='signvid_ase.ogv' type='video/ogg; codecs="theora"'
                     media='desktop' lang='ase' role='sign' >
    <source id='sign2' src='signvid_gsg.ogv' type='video/ogg; codecs="theora"'
                     media='desktop' lang='gsg' role='sign' >
    <source id='sign3' src='signvid_sfs.ogv' type='video/ogg; codecs="theora"'
                     media='desktop' lang='sfs' role='sign' >
    <source id='tad1' src='tad_en.srt' type='text/srt; charset="ISO-8859-1"'
                     lang='en' role='tad' >
    <source id='tad2' src='video.ogv?track=tad[de]' type='text/srt; charset="ISO-8859-1"'
                     lang='de' role='tad' >
    <source id='tad3' src='tad_ja.srt' type='text/srt; charset="EUC-JP"' lang='ja'
                     role='tad' >

Note that this somewhat ignores my previously proposed special itext tag for handling text tracks. I am doing this here to experiment with a more integrative approach with the virtual media resource idea from the previous post. This may well be a better solution than a specific new text-related element. Most of the attributes of the itext element are, incidentally, covered.

You will also notice that some of the tracks are references to tracks inside binary media files using the Media Fragment URI specification while others link to full files. An example is video.ogv?track=auddesc[en]. So, this is a uniform means of exposing all the tracks that are part of a (virtual) media resource to the UA, no matter whether in-band or in external files. It actually relies on the UA or server being able to resolve these URLs.

“type” attribute

“media” and “type” are existing attributes of the <source> element in HTML5 and meant to help the UA determine what to do with the referenced resource. The current spec states:

The “type” attribute gives the type of the media resource, to help the user agent determine if it can play this media resource before fetching it.

The word “play” might need to be replaced with “decode” to cover several different MIME types.

The “type” attribute was also extended with the possibility to add the “charset” MIME parameter of a linked text resource – this is particularly important for SRT files, which don’t handle charsets very well. It avoids having to add an additional attribute and is analogous to the “codecs” MIME parameter used by audio and video resources.

“media” attribute

Further, the spec states:

The “media” attribute gives the intended media type of the media resource, to help the user agent determine if this media resource is useful to the user before fetching it. Its value must be a valid media query.

The “mobile” and “desktop” values are hints that I’ve used for simplicity reasons. They could be improved by giving appropriate bandwidth limits and width/height values, etc. Other values could be different camera angles such as topview, frontview, backview. The media query aspect has to be looked into in more depth.

“lang” attribute

The above example further uses “lang” and “role” attributes:

The “lang” attribute is an existing global attribute of HTML5, which typically indicates the language of the data inside the element. Here, it is used to indicate the language of the referenced resource. This is possibly not quite the best name choice and should maybe be called “hreflang”, which is already used in multiple other elements to signify the language of the referenced resource.

“role” attribute

The “role” attribute is also an existing attribute in HTML5, included from ARIA. It currently doesn’t cover media resources, but could be extended. The suggestion here is to specify the roles of the different media tracks – the ones I have used here are:

  • “media”: a main media resource – typically contains audio and video and possibly more
  • “dub”: a audio track that provides an alternative dubbed language track
  • “auddesc”: a audio track that provides an additional audio description track
  • “caption”: a text track that provides captions
  • “sign”: a video-only track that provides an additional sign language video track
  • “tad”: a text track that provides textual audio descriptions to be read by a screen reader or a braille device

Further roles could be “music”, “speech”, “sfx” for audio tracks, “subtitle”, “lyrics”, “annotation”, “chapters”, “overlay” for text tracks, and “alternate” for a alternate main media resource, e.g. a different camera angle.

Track activation

The given attributes help the UA decide what to display.

It will firstly find out from the “type” attribute if it is capable of decoding the track.

Then, the UA will find out from the “media” query, “role”, and “lang” attributes whether a track is relevant to its user. This will require checking the capabilities of the device, network, and the user preferences.

Further, it could be possible for Web authors to influence whether a track is displayed or not through CSS parameters on the <source> element: “display: none” or “visibility: hidden/visible”.

Examples for track activation that a UA would undertake using the example above:

Given a desktop computer with Firefox, German language preferences, captions and sign language activated, the UA will fetch the original video at video.ogv (for Firefox), the German caption track at video.ogv?track=caption[de], and the German sign language track at signvid_gsg.ogv (maybe also the German dubbed audio track at video.ogv?track=audio[de], which would then replace the original one).

Given a desktop computer with Safari, English language preferences and audio descriptions activated, the UA will fetch the original video at video.mp4 (for Safari) and the textual audio description at tad_en.srt to be displayed through the screen reader, since it cannot decode the Ogg audio description track at video.ogv?track=auddesc[en].

Also, all decodeable tracks could be exposed in a right-click menu and added on-demand.

Display styling

Default styling of these tracks could be:

  • video or alternate video in the video display area,
  • sign language probably as picture-in-picture (making it useless on a mobile and only of limited use on the desktop),
  • captions/subtitles/lyrics as overlays on the bottom of the video display area (or whatever the caption format prescribes),
  • textual audio descriptions as ARIA live regions hidden behind the video or off-screen.

Multiple audio tracks can always be played at the same time.

The Web author could also define the display area for a track through CSS styling and the UA would then render the data into that area at the rate that is required by the track.

How good is this approach?

The advantage of this new proposal is that it builds basically on existing HTML5 components with minimal additions to satisfy requirements for content selection and accessibility of media elements. It is a declarative approach to the multi-track media resource challenge.

However, it leaves most of the decision on what tracks are alternatives of/additions to each other and which tracks should be displayed to the UA. The UA makes an informed decision because it gets a lot of information through the attributes, but it still has to make decisions that may become rather complex. Maybe there needs to be a grouping level for alternative tracks and additional tracks – similar to what I did with the second itext proposal, or similar to the <switch> and <par> elements of SMIL.

A further issue is one that is currently being discussed within the Media Fragments WG: how can you discover the track composition and the track naming/uses of a particular media resource? How, e.g., can a Web author on another Web site know how to address the tracks inside your binary media resource? A HTML specification like the above can help. But what if that doesn’t exist? And what if the file is being used offline?

Alternative Manifest descriptions

The need to manifest the track composition of a media resource is not a new one. Many other formats and applications had to deal with these challenges before – some have defined and published their format.

I am going to list a few of these formats here with examples. They could inspire a next version of the above proposal with grouping elements.

Microsoft ISM files (SMIL subpart)

With the release of IIS7, Microsoft introduced “Smooth Streaming”, which uses chunking on files on the server to deliver adaptive streaming to Silverlight clients over HTTP. To inform a smooth streaming client of the tracks available for a media resource, Microsoft defined ism files: IIS Smooth Streaming Server Manifest files.

This is a short example – a longer one can be found here:

<?xml version=

New proposal for captions and other timed text for HTML5

The first specification for how to include captions, subtitles, lyrics, and similar time-aligned text with HTML5 media elements has received a lot of feedback – probably because there are several demos available.

The feedback has encouraged me to develop a new specification that includes the concerns and makes it easier to associate out-of-band time-aligned text (i.e. subtitles stored in separate files to the video/audio file). A simple example of the new specification using srt files is this:

<video src="video.ogv" controls>
   <itextlist category="CC">
     <itext src="caption_en.srt" lang="en"/>
     <itext src="caption_de.srt" lang="de"/>
     <itext src="caption_fr.srt" lang="fr"/>
     <itext src="caption_jp.srt" lang="jp"/>

By default, the charset of the itext file is UTF-8, and the default format is text/srt (incidentally a mime type the still needs to be registered). Also by default the browser is expected to select for display the track that matches the set default language of the browser. This has been proven to work well in the previous experiments.

Check out the new itext specification, read on to get an introduction to what has changed, and leave me your feedback if you can!

The itextlist element
You will have noticed that in comparison to the previous specification, this specification contains a grouping element called “itextlist”. This is necessary because we have to distinguish between alternative time-aligned text tracks and ones that can be additional, i.e. displayed at the same time. In the first specification this was done by inspecting each itext element’s category and grouping them together, but that resulted in much repetition and unreadable specifications.

Also, it was not clear which itext elements were to be displayed in the same region and which in different ones. Now, their styling can be controlled uniformly.

The final advantage is that association of callbacks for entering and leaving text segments as extracted from the itext elements can now be controlled from the itextlist element in a uniform manner.

This change also makes it simple for a parser to determine the structure of the menu that is created and included in the controls element of the audio or video element.

Incidentally, a patch for Firefox already exists that makes this part of the browser. It does not yet support this new itext specification, but here is a screenshot that Felipe Corr

HTML5 audio element accessibility

As part of my experiments in video accessibility I am also looking at the audio element. I have just finished a proof of concept for parsing Lyrics files for music in lrc format.

The demo uses Tay Zonday’s “Chocolate Rain” song both as a video with subtitles and as an audio file with lyrics. Fortunately, he published these all under a creative commons license, so I was able to use this music file. BTW: I found it really difficult to find a openly licensed music file with lyrics.

While I was at it, I also cleaned up all the old demos and now have a nice list of all demos in a central file.

ARIA – A Brief Introduction

Since working on video accessibility, I have felt rather inadequate not knowing exactly how general Web accessibility works, in particular ARIA. I have been pointed at the W3C WAI-ARIA primer, best practices, and WD specification, but found them almost impossible to read.

If you are looking for a document that gets right to the point, I can recommend Opera’s Introduction to WAI ARIA. It tells you what attributes there are and how to use them. More in-depth information is available in the W3C WAI-ARIA best practices. Here’s my little summary of what I learnt.

Getting straight to the point: ARIA mostly cares about giving screen control to the keyboard (away from the mouse) and about exposing semantic information, such that vision-impaired people have a way to interact with Web content and screen readers can read out useful information.

Basic keys
The basic keys in use for accessibility are the tab/shift+tab, arrow, enter, space and escape keys.

Keyboard Focus: tabbing
Normal tabbing includes form controls and anchors. This can be overruled with the tabindex attribute.

Adding a tabindex=0 to an element adds the element to the tab order in which it appears in the document. Adding a tabindex out of [1;32767] you can place any element into a desired order – lowest numbers first.

Adding a tabindex=-1 to an element removes it from tabbing order, but you can still get keyboard focus onto it through javascript, e.g. for the subelements of a menu. The aria-activedescendant attribute can tell which is active in a list of descendants.

Navigation Landmarks: roles
Screenreaders have a problem with expressing what the functionality of elements is – normally they can only read out the name of the element.

This is where the role attribute comes in. It provides semantic meaning, e.g. “slider” instead of “input” element.

ARIA has a large number of pre-defined roles. They are listed in the spec – each role has additional attributes to provide more assistive information – mostly state information on the particular element.

Live updated content: aria-live
When data is updated somewhere on screen, often assistive technology doesn’t get to know about it.

Regions that are marked with the aria-live attribute will be read out even if the user is focused on another part of the screen at that point.

Form input: aria-reqired
For screen readers it is not obvious if a form element’s entry is a required or optional entry. Add an aria-required attribute to the form entry element and your screen reader will tell you.

Labels and descriptions: aria-labelledby / aria-describedby
Most often the description or label for a page area sits already elsewhere on screen, but with no obivous relationship to an element other than visible neighbourship.

A screenreader can be told about the relationship by using the aria-labelledby / aria-describedby attributes, which allow to link to such an area through that area’s id attribute.

Is that all?
Yes, I think that’s essentially all. It’s not particularly difficult, but it has a high impact on accessibility. I hope your take-away is as big as mine!

BTW: WAI ARIA is written for good old HTML4, not HTML5. However, there are synchronisation activities under way and WAI ARIA attributes will still be relevant to HTML5. Some of the roles will become unnecessary with the new elements available in HTML5 – see a draft mapping of HTML5 elements to ARIA implicit roles in Henry’s excellent document, but it seems the tabbing order, live regions, and the role attribute are here to stay.

More video accessibility work

It’s already old news, but I am really excited about having started a new part-time contract with Mozilla to continue pushing the HTML5 video and audio elements towards accessibility.

My aim is two-fold: firstly to improve the HTML5 audio and video tags with textual representations, and secondly to hook up the Ogg file format with these accessibility features through an Ogg-internal text codec.

The textual representation that I am after is closely based on the itext elements I have been proposing for a while. They are meant to be a simple way to associate external subtitle/caption files with the HTML5 video and audio tags. I am initially looking at srt and DFXP formats, because I think they are extremes of a spectrum of time-aligned text formats from simple to complex. I am preparing a specification and javascript demonstration of the itext feature and will then be looking for constructive criticism from accessibility, captioning, Web, video and any other expert who cares to provide input. My hope is to move the caption discussion forward on the WHATWG and ultimately achieve a cross-browser standard means for associating time-aligned text with media streams.

The Ogg-internal solution for subtitles – and more generally for time-aligned text – is then a logical next step towards solving accessibility. From the many discussions I have had on the topic of how best to associate subtitles with video I have learnt that there is a need for both: external text files with subtitles, as well as subtitles that are multiplexed with the media into a single binary fie. Here, I am particularly looking at the Kate codec as a means of multiplexing srt and DFXP into Ogg.

Eventually, the idea is to have a seamless interface in the Web Browser for dealing with subtitles, captions, karaoke, timed metadata, and similar time-aligned text. The user interaction should be identical no matter whether the text comes from within a binary media file or from a secondary Web resource. Once this seamless interface exists, hooking up accessibility tools such as screen readers or braille devices to the data should in theory be simple.

A review of the W3C Timed Text Authoring Format

UPDATE: The best demo I have seen so far of many of DFXP’s features is at http://www.w3.org/2009/02/ThisIsCoffee.html.

The W3C has published a third last call for the draft specification of DFXP, the Distribution Format Exchange Profile for the Timed Text Authoring Format – or short: for their new standard format for captions. Comments are due by the 30th June, so rush if you want to give any feedback. Here is what came to my mind as I was reading the 183 pages long document.

Please note: This review looks at DFXP from a Web view, i.e. how compatible is it with existing Web technologies, since my main use case will be on the Web, even if advocates will say that that’s not it’s main purpose, strangely enough, for a standard coming out of the W3C.

The state of affairs with caption formats

When it comes to caption and subtitles, there is no lack of formats. It seems, because it is an easy challenge to define a data format for something as simple as a piece of text and some timing information, every new project that wanted to deal with captions – or more generally timed text – created their own format. I am no exception to the rule. 🙂

Thus, the current state of affairs wrt timed text is that there are many different textual file formats to store such data, there are also many different video container formats each with their own data format (or even formats) for embedding timed text into them, and there is a lot of software that will deal with many input, output and encapsulation formats.

The problem with this situation is that the formats are all different in their complexity. The simple “piece of text and timing information” problem can be turned into as complex a problem as you desire. By adding layout information, styling information, animation functionality, metadata about the video and about the content, and possibly hyperlinks, we have ended up in a large mess of incompatible formats.

The aim of W3C Timed Text

The W3C Timed Text working group was chartered in January 2003 to attack this issue. It was supposed to become the super-format of all possible functionalities for timed text formats and therefore a perfect interchange format between applications (see requirements document). Its focus was for use on the Web and with SMIL and to make use of existing W3C technologies where possible

However, the history of captioning is TV and the scope of Timed Text is beyond mere use on the Web, so while W3C Timed Text took a lot of inspiration from other Web standards, it has become a stand-alone standard that does not rely on, e.g. the availability of a CSS engine, and it has no in-built hyperlinking functionality (see what requirements it fulfills).

Dissecting DFXP

So. let’s look into some of what DFXP provides.

Here is an example file taken straight from the draft – check the presentation here:

<tt xml:lang="" xmlns="http://www.w3.org/2006/10/ttaf1">
    <metadata xmlns:ttm="http://www.w3.org/2006/10/ttaf1#metadata">
      <ttm:title>Timed Text DFXP Example</ttm:title>
      <ttm:copyright>The Authors (c) 2006</ttm:copyright>

    <styling xmlns:tts="http://www.w3.org/2006/10/ttaf1#styling">
      <!-- s1 specifies default color, font, and text alignment -->
      <style xml:id="s1"
                 tts:textAlign="center" />
      <!-- alternative using yellow text but otherwise the same as style s1 -->
      <style xml:id="s2" style="s1" tts:color="yellow"/>
      <!-- a style based on s1 but justified to the right -->
      <style xml:id="s1Right" style="s1" tts:textAlign="end" />     
      <!-- a style based on s2 but justified to the left -->
      <style xml:id="s2Left" style="s2" tts:textAlign="start" />

    <layout xmlns:tts="http://www.w3.org/2006/10/ttaf1#styling">
      <region xml:id="subtitleArea"
                   tts:extent="560px 62px"
                   tts:padding="5px 3px"
                   tts:displayAlign="after" />
  <body region="subtitleArea">
      <p xml:id="subtitle1" begin="0.76s" end="3.45s">
        It seems a paradox, does it not,
      <p xml:id="subtitle2" begin="5.0s" end="10.0s">
        that the image formed on<br/>
        the Retina should be inverted?
      <p xml:id="subtitle3" begin="10.0s" end="16.0s" style="s2">
        It is puzzling, why is it<br/>
        we do not see things upside-down?
      <p xml:id="subtitle4" begin="17.2s" end="23.0s">
        You have never heard the Theory,<br/>
        then, that the Brain also is inverted?
      <p xml:id="subtitle5" begin="23.0s" end="27.0s" style="s2">
        No indeed! What a beautiful fact!
      <p xml:id="subtitle6a" begin="28.0s" end="34.6s" style="s2Left">
        But how is it proved?
      <p xml:id="subtitle6b" begin="28.0s" end="34.6s" style="s1Right">
        Thus: what we call
      <p xml:id="subtitle7" begin="34.6s" end="45.0s" style="s1Right">
        the vertex of the Brain<br/>
        is really its base
      <p xml:id="subtitle8" begin="45.0s" end="52.0s" style="s1Right">
        and what we call its base<br/>
        is really its vertex,
      <p xml:id="subtitle9a" begin="53.5s" end="58.7s">
        it is simply a question of nomenclature.
      <p xml:id="subtitle9b" begin="53.5s" end="58.7s" style="s2">
        How truly delightful!

I’m going to look at each of the different functionalities separately and discuss their strengths and weaknesses.


Let’s begin with the body of the DFXP document and what elements are defined for this area.

Firstly, <body> comes with optional begin, end, and dur attributes. As is the case for all time specifications in DFXP, there are both “end” and “dur” attributes. Why this over-specification? There is not even an explanation which of the two has higher priority when in conflict. This is plainly asking for trouble – why not simplify the spec?

The “region” and “style” attributes refer to a previously defined region and styles that are applied to the body. “id” and “lang” attributes allow to associate a name and a language with the body.

The “timeContainer” attribute enables the author to specify whether the elements in the body are all to be regarded as temporally parallel or in sequence, the default being parallel. This means that all text elements specified inside the body can render over the top of each other – a situation that is solved by giving them specific start and end times.

The containing elements of body are a sequence of <div> tags. The div element functions as a logical container and a temporal structuring element for a sequence of textual content units. div elements like body elements are allowed a “start”, “end” and “dur” attribute and generally everything that the body element also has, except that their children can be more div or p. Again, the children of the div element are all regarded as being temporally parallel.

The p element is basically the inner-most element that contains the actual text, including new-lines (br) and spans to associate further styling, metadata, or animations. The children of the p or span element are also all regarded as being temporally parallel, unless otherwise specified.

The structuring of text into div, p, and span elements seems to make sense and provide sufficient (if not even excessive) flexibility for any required timed text needs.


Once the text is specified and structured, the next question is where it should be positioned.

The extent attribute of the <tt> root element specifies the width and height of the root container, if not specified by the external authoring context.

Inside the root container, regions are defined through explicit <region> elements. The origin of placement for a region is the top left corner. Regions can define their “origin” offset, their “width” and “height”, the alignment of text within them through the “textAlign” and “displayAlign” styles, and whether text that “overflows” a region should be visible or hidden.

The way in which DFXP defines regions and placement of text within regions is very different to the way in which HTML and CSS work. By default, elements in HTML flow one after another in the same order as they appear in the source. CSS attributes applied to the elements can control their positioning through giving coordinates, or relative placements in relation to other elements. In DFXP elements are placed inside regions that are styled, making it incompatible with HTML.


The styling attributes available for DFXP are limited, but sufficient for timed text purposes. The way in which style associations to elements are resolved is quite diverse. Styles can be associated with regions, with individual elements, individually and as a group, through layouts and through parent elements. Compared to CSS, it feels complicated and potentially full of contradictions.


Further to styling, DFXP defines animations, which are discrete changes to some style parameter value that applies over some time interval. This is relevant for example to implement karaoke style colouring of text over time.


The <metadata> element serves as a generic container for grouping metadata information. It can be associated virtually with any element – which seems somewhat over-flexible, but provides for interesting meta data information such as meta data for styles or for a <br>.

In addition, metadata is actually limited to a set number of elements: title, desc, copyright, agent, name, and actor. These are strange fields – in particular if you compare them to the flexibility of HTML meta data, which consists of free-form name-value pairs, bringing us domain-specific schemes such as the Dublin Core. This is not easily possible here, but instead one has to define extensions to allow for such flexible meta data.

Other features

DFXP provides other features such as information that describes the related video file, e.g. frameRate, subFrameRate, frameRateMultiplier, pixelAspectRatio, smpteMode, timeBase, and tickRate. Such information will help at the point in time when DFXP is supposed to be multiplexed into a binary media file together with audio and video tracks. These attributes can provide information required for the multiplexing process. I am not sure that justifies their existence though.

Other, minor features are available too. Check out the full specification to get a complete picture.


Part of the publication of this draft is also a test suite. Several of the defined features are still not represented in the test suite, which to me raises the question if they are really required. It might do wonders to the draft size to remove them.


DFXP is a standard for timed text that is firmly grounded in past captioning specifications, but written in XML, and borrowing ideas from Web technologies. It is unfortunately not re-using existing Web infrastructure to implement its more complex features: no use of CSS for styling and layout, no use of hyperlinks. Also, the use of namespaces seems excessive and won’t make it easy to author this format, in particular since the defined namespaces do not map into the defined profiles.

DFXP is, however, simple to transcode to something that a Web Browser can deal with through its existing engines, because it has borrowed from other Web standards. It is thus easier to work with on the Web than most other formats. It should be relatively easy to map to HTML, CSS and javascript, as already started in the test suite with the HTML5 video element.

DFXP is witten in such a way that it is possible to put together a new profile with extensions that are more appropriate for specific needs, e.g. that fit better into existing Web infrastructure. Currently, DFXP has three defined profiles: one focused on transformation, one focused on presentation, and one that contains everything.

I think it’s time for a html5 profile of DFXP that at minimum extends DFXP with hyperlinks, making it a real timed text Web format.