Tag Archives: video accessibility

WebVTT as a W3C Recommendation

Three weeks ago I attended TPAC, the annual meeting of W3C Working Groups. One of the meetings was of the Timed Text Working Group (TT-WG), that has been specifying TTML, the Timed Text Markup Language. It is now proposed that WebVTT be also standardised through the same Working Group.

How did that happen, you may ask, in particular since WebVTT and TTML have in the past been portrayed as rival caption formats? How will the WebVTT spec that is currently under development in the Text Track Community Group (TT-CG) move through a Working Group process?

I’ll explain first why there is a need for WebVTT to become a W3C Recommendation, and then how this is proposed to be part of the Timed Text Working Group deliverables, and finally how I can see this working between the TT-CG and the TT-WG.

Advantages of a W3C Recommendation

TTML is a XML-based markup format for captions developed during the time that XML was all the hotness. It has become a W3C standard (a so-called “Recommendation”) despite not having been implemented in any browsers (if you ask me: that’s actually a flaw of the W3C standardisation process: it requires only two interoperable implementations of any kind – and that could be anyone’s JavaScript library or Flash demonstrator – it doesn’t actually require browser implementations. But I digress…). To be fair, a subpart of TTML is by now implemented in Internet Explorer, but all the other major browsers have thus far rejected proposals of implementation.

Because of its Recommendation status, TTML has become the basis for several other caption standards that other SDOs have picked: the SMPTE’s SMPTE-TT format, the EBU’s EBU-TT format, and the DASH Industry Forum’s use of SMPTE-TT. SMPTE-TT has also become the “safe harbour” format for the US legislation on captioning as decided by the FCC. (Note that the FCC requirements for captions on the Web are actually based on a list of features rather than requiring a specific format. But that will be the topic of a different blog post…)

WebVTT is much younger than TTML. TTML was developed as an interchange format among caption authoring systems. WebVTT was built for rendering in Web browsers and with HTML5 in mind. It meets the requirements of the <track> element and supports more than just captions/subtitles. WebVTT is popular with browser developers and has already been implemented in all major browsers (Firefox Nightly is the last to implement it – all others have support already released).

As we can see and as has been proven by the HTML spec and multiple other specs: browsers don’t wait for specifications to have W3C Recommendation status before they implement them. Nor do they really care about the status of a spec – what they care about is whether a spec makes sense for the Web developer and user communities and whether it fits in the Web platform. WebVTT has obviously achieved this status, even with an evolving spec. (Note that the spec tries very hard not to break backwards compatibility, thus all past implementations will at least be compatible with the more basic features of the spec.)

Given that Web browsers don’t need WebVTT to become a W3C standard, why then should we spend effort in moving the spec through the W3C process to become a W3C Recommendation?

The modern Web is now much bigger than just Web browsers. Web specifications are being used in all kinds of devices including TV set-top boxes, phone and tablet apps, and even unexpected devices such as white goods. Videos are increasingly omnipresent thus exposing deaf and hard-of-hearing users to ever-growing challenges in interacting with content on diverse devices. Some of these devices will not use auto-updating software but fixed versions so can’t easily adapt to new features. Thus, caption producers (both commercial and community) need to be able to author captions (and other video accessibility content as defined by the HTML5 element) towards a feature set that is clearly defined to be supported by such non-updating devices.

Understandably, device vendors in this space have a need to build their technology on standardised specifications. SDOs for such device technologies like to reference fixed specifications so the feature set is not continually updating. To reference WebVTT, they could use a snapshot of the specification at any time and reference that, but that’s not how SDOs work. They prefer referencing an officially sanctioned and tested version of a specification – for a W3C specification that means creating a W3C Recommendation of the WebVTT spec.

Taking WebVTT on a W3C recommendation track is actually advantageous for browsers, too, because a test suite will have to be developed that proves that features are implemented in an interoperable manner. In summary, I can see the advantages and personally support the effort to take WebVTT through to a W3C Recommendation.

Choice of Working Group

FAIK this is the first time that a specification developed in a Community Group is being moved into the recommendation track. This is something that has been expected when the W3C created CGs, but not something that has an established process yet.

The first question of course is which WG would take it through to Recommendation? Would we create a new Working Group or find an existing one to move the specification through? Since WGs involve a lot of overhead, the preference was to add WebVTT to the charter of an existing WG. The two obvious candidates were the HTML WG and the TT-WG – the first because it’s where WebVTT originated and the latter because it’s the closest thematically.

Adding a deliverable to a WG is a major undertaking. The TT-WG is currently in the process of re-chartering and thus a suggestion was made to add WebVTT to the milestones of this WG. TBH that was not my first choice. Since I’m already an editor in the HTML WG and WebVTT is very closely related to HTML and can be tested extensively as part of HTML, I preferred the HTML WG. However, adding WebVTT to the TT-WG has some advantages, too.

Since TTML is an exchange format, lots of captions that will be created (at least professionally) will be in TTML and TTML-related formats. It makes sense to create a mapping from TTML to WebVTT for rendering in browsers. The expertise of both, TTML and WebVTT experts is required to develop a good mapping – as has been shown when we developed the mapping from CEA608/708 to WebVTT. Also, captioning experts are already in the TT-WG, so it helps to get a second set of eyes onto WebVTT.

A disadvantage of moving a specification out of a CG into a WG is, however, that you potentially lose a lot of the expertise that is already involved in the development of the spec. People don’t easily re-subscribe to additional mailing lists or want the additional complexity of involving another community (see e.g. this email).

So, a good process needs to be developed to allow everyone to contribute to the spec in the best way possible without requiring duplicate work. How can we do that?

The forthcoming process

At TPAC the TT-WG discussed for several hours what the next steps are in taking WebVTT through the TT-WG to recommendation status (agenda with slides). I won’t bore you with the different views – if you are keen, you can read the minutes.

What I came away with is the following process:

  1. Fix a few more bugs in the CG until we’re happy with the feature set in the CG. This should match the feature set that we realistically expect devices to implement for a first version of the WebVTT spec.
  2. Make a FSA (Final Specification Agreement) in the CG to create a stable reference and a clean IPR position.
  3. Assuming that the TT-WG’s charter has been approved with WebVTT as a milestone, we would next bring the FSA specification into the TT-WG as FPWD (First Public Working Draft) and immediately do a Last Call which effectively freezes the feature set (this is possible because there has already been wide community review of the WebVTT spec); in parallel, the CG can continue to develop the next version of the WebVTT spec with new features (just like it is happening with the HTML5 and HTML5.1 specifications).
  4. Develop a test suite and address any issues in the Last Call document (of course, also fix these issues in the CG version of the spec).
  5. As per W3C process, substantive and minor changes to Last Call documents have to be reported and raised issues addressed before the spec can progress to the next level: Candidate Recommendation status.
  6. For the next step – Proposed Recommendation status – an implementation report is necessary, and thus the test suite needs to be finalized for the given feature set. The feature set may also be reduced at this stage to just the ones implemented interoperably, leaving any other features for the next version of the spec.
  7. The final step is Recommendation status, which simply requires sufficient support and endorsement by W3C members.

The first version of the WebVTT spec naturally has a focus on captioning (and subtitling), since this has been the dominant use case that we have focused on this far and it’s the part that is the most compatibly implemented feature set of WebVTT in browsers. It’s my expectation that the next version of WebVTT will have a lot more features related to audio descriptions, chapters and metadata. Thus, this seems a good time for a first version feature freeze.

There are still several obstacles towards progressing WebVTT as a milestone of the TT-WG. Apart from the need to get buy-in from the TT-WG, the TT-CG, and the AC (Adivisory Committee who have to approve the new charter), we’re also looking at the license of the specification document.

The CG specification has an open license that allows creating derivative work as long as there is attribution, while the W3C document license for documents on the recommendation track does not allow the creation of derivative work unless given explicit exceptions. This is an issue that is currently being discussed in the W3C with a proposal for a CC-BY license on the Recommendation track. However, my view is that it’s probably ok to use the different document licenses: the TT-WG will work on WebVTT 1.0 and give it a W3C document license, while the CG starts working on the next WebVTT version under the open CG license. It probably actually makes sense to have a less open license on a frozen spec.

Making the best of a complicated world

WebVTT is now proposed as part of the recharter of the TT-WG. I have no idea how complicated the process will become to achieve a W3C WebVTT 1.0 Recommendation, but I am hoping that what is outlined above will be workable in such a way that all of us get to focus on progressing the technology.

At TPAC I got the impression that the TT-WG is committed to progressing WebVTT to Recommendation status. I know that the TT-CG is committed to continue developing WebVTT to its full potential for all kinds of media-time aligned content with new kinds already discussed at FOMS. Let’s enable both groups to achieve their goals. As a consequence, we will allow the two formats to excel where they do: TTML as an interchange format and WebVTT as a browser rendering format.

Summary Video Accessibility Talk

I’ve just got off a call to the UK Digital TV Group, for which I gave a talk on HTML5 video accessibility (slides best viewed in Google Chrome).

The slide provide a high-level summary of the accessibility features that we’ve developed in the W3C for HTML5, including:

  • Subtitles & Captions with WebVTT and the track element
  • Video Descriptions with WebVTT, the track element and speech synthesis
  • Chapters with WebVTT for semantic navigation
  • Audio Descriptions through synchronising an audio track with a video
  • Sign Language video synchronized with a main video

I received some excellent questions.

The obvious one was about why WebVTT and not TTML. While for anyone who has tried to implement TTML support, the advantages of WebVTT should be clear, for some the decision of the browsers to go with WebVTT still seems to be bothersome. The advantages of CSS over XSL-FO in a browser-context are obvious, but not as much outside browsers. So, the simplicity of WebVTT and the clear integration with HTML have to speak for themselves. Conversion between TTML and WebVTT was a feature that was being asked for.

I received a question about how to support ducking (reduce the volume of the main audio track) when using video descriptions. My reply was to either use video descriptions with WebVTT and do ducking during the times that a cue is active, or when using audio descriptions (i.e. actual audio tracks) to add an additional WebVTT file of kind=metadata to mark the intervals in which to do ducking. In both cases some JavaScript will be necessary.

I received another question about how to do clean audio, which I had almost forgotten was a requirement from our earlier media accessibility document. “Clean audio” consists of isolating the audio channel containing the spoken dialog and important non-speech information that can then be amplified or otherwise modified, while other channels containing music or ambient sounds are attenuated. I suggested using the mediagroup attribute to provide a main video element (without an audio track) and then the other channels as parallel audio tracks that can be turned on and off and attenuated individually. There is some JavaScript coding involved on top of the APIs that we have defined in HTML, but it can be implemented in browsers that support the mediagroup attribute.

Another question was about the possibilities to extend the list of @kind attribute values. I explained that right now we have a proposal for a new text track kind=”forced” so as to provide forced subtitles for sections of video with foreign language. These would be on when no other subtitle or caption tracks are activated. I also explained that if there is a need for application-specific text tracks, the kind=”metadata” would be the correct choice.

I received some further questions, in particular about how to apply styling to captions (e.g. color changes to text) and about how closely the browser are able to keep synchronization across multiple media elements. The earlier was easily answered with the ::cue pseudo-element, but the latter is a quality of implementation feature, so I had to defer to individual browsers.

Overall it was a good exercise to summarize the current state of HTML5 video accessibility and I was excited to show off support in Chrome for all the features that we designed into the standard.

My crazy linux.conf.au week

In January I attended the annual Australian Linux and Open Source conference (LCA). But since I was sick all of January and had a lot to catch up on, I never got around to sharing all the talks that I gave during that time.

Drupal Down Under

It started with a talk at Drupal Down Under, which happened the weekend before LCA. I gave a talk titled “HTML5 video specifications” (video, slides).

I spoke about the video and audio element in HTML5, how to provide fallback content, how to encode content, how to control them from JavaScript, and briefly about Drupal video modules, though the next presentation provided much more insight into those. I explained how to make the HTML5 media elements accessible, including accessible controls, captions, audio descriptions, and the new WebVTT file format. I ran out of time to introduce the last section of my slides which are on WebRTC.


On the first day of LCA I gave a talk both in the Multimedia Miniconf and the Browser Miniconf.

Browser Miniconf

In the Browser Miniconf I talked about “Web Standardisation – how browser vendors collaborate, or not” (slides). Maybe the most interesting part about this was that I tried out a new slide “deck” tool called impress.js. I’m not yet sure if I like it but it worked well for this talk, in which I explained how the HTML5 spec is authored and who has input.

I also sat on a panel of browser developers in the Browser Miniconf (more as a standards than as a browser developer, but that’s close enough). We were asked about all kinds of latest developments in HTML5, CSS3, and media standards in the browser.

Multimedia Miniconf

In the Multimedia Miniconf I gave a “HTML5 media accessibility update” (slides). I talked about the accessibility problems of Flash, how native HTML5 video players will be better, about accessible video controls, captions, navigation chapters, audio descriptions, and WebVTT. I also provided a demo of how to synchronize multiple video elements using a polyfill for the multitrack API.

I also provided an update on HTTP adaptive streaming APIs as a lightning talk in the Multimedia Miniconf. I used an extract of the Drupal conference slides for it.

Main conference

Finally, and most importantly, Alice Boxhall and myself gave a talk in the main linux.conf.au titled “Developing Accessible Web Apps – how hard can it be?” (video, slides). I spoke about a process that you can follow to make your Web applications accessible. I’m writing a separate blog post to explain this in more detail. In her part, Alice dug below the surface of browsers to explain how the accessibility markup that Web developers provide is transformed into data structures that are handed to accessibility technologies.

3rd W3C Web and TV Workshop, Hollywood

Curious about any new requirements that the TV community may have for HTML5 video, I attended the W3C Web and TV Workshop in Hollywood last week. It’s already the third of its kind and was also the largest to date showing an increasing interest of the TV community to converge with the Web community.

The Workshop Aim

I went into the Workshop not quite knowing what to expect. My previous contact with members of this community was restricted to email exchanges on the W3C Web and TV Interest Group (IG) mailing list. I knew there was some interest in video accessibility (well: particularly captions) and little knowledge of existing HTML5 specifications around text tracks and why the browsers were going with WebVTT. So I had decided to attend the workshop to get a better understanding of the community, it’s background, needs, and issues, and to hopefully teach some of the ways of HTML5. For that reason I had also submitted a WebVTT presentation/demo.

As it turned out, the workshop had as its key target the facilitation of communication between the TV and the HTML5 community. The aim was to identify features that need to be added to the HTML5 video element to satisfy the needs of the TV community. I obviously came to the right workshop.

The process that is being used by the W3C in the Interest Group is to have TV community members express their needs, then have HTML5 experts express how these needs can be satisfied with existing HTML5 features, then make trial implementations and identify any shortcomings, then move forward to progress these through HTML5 or HTML.next. This workshop clearly focused on the first step: expressing needs.

Often times it was painful for me to watch presenters defending their requirements and trying to impress on the audience how important a certain feature is to them when that features actually already has a HTML5 specification, but just not yet a browser implementations. That there were so few HTML5 video experts present and that they were given very little space to directly reply to the expressed needs and actually explain what is already possible (or specified to be possible) was probably one of the biggest drawbacks of the workshop.

To be fair, detailed technical discussions were not possible in a room with 150 attendees with a panel sitting at the front discussing topics and taking questions. Solving a use case with existing HTML5 markup and identifying the gaps requires smaller break-out groups of a maximum of maybe 20 people and sufficient HTML5 knowledge in the room. Ultimately they require a single person to try to implement it using JavaScript alone, and, failing that, writing browser extensions. Only such code actually proves that a feature is missing.

Now, the video features of HTML5 are still continuing to change almost on a daily basis. Much development is, for example, happening around real-time communication features and around the track element as we speak. So, focusing on further requirements finding around HTML5 video for now is probably a good thing.

The TV Community Approach

Before I move on to some of the topics covered by the workshop, I have to express some concern about the behaviour that I observed with lots of the TV community folks. Many people tried pushing existing solutions from other spaces into the Web unchanged with a claim of not re-inventing the wheel and following paved cowpaths, which are some of the underlying design principles for HTML5. I can understand where such behaviour originates thinking that having solved the same problems elsewhere before, those solutions should apply here, too. But I would like to warn people of this approach.

If we blindly apply solutions that were not developed for HTML5 into HTML we will end up with suboptimal solutions that will hurt us further down the track. The principles of not re-inventing the wheel and following paved cowpaths were introduced for features that were already implemented by browsers or in de-facto standard use by JavaScript libraries. They were not created for new features in HTML. The video element is a completely new feature in HTML thus everything around it is new.

I would therefore like to see some more respect given to HTML5 and the complexities involved in finding the best possible technical solutions for the Web given that the video element does not stand alone in HTML5, but is part of a much larger picture of technical capabilities on the Web where many of the requested features for TV applications may already be solved by existing HTML markup that is not part of the video element.

Also, HTML5 is not just about the HTML markup, but also about CSS and JavaScript and HTTP. There are several layers of technology involved in creating a Web application: not only a separation of work between client and servers, but also between the Operating System, the media framework, the browser, browser plugins, and JavaScript has to be balanced. To get this balance right is a fine art that will take many discussion, many experiments and sometimes several design approaches. We need patience and calm to work through this, not a rushed adoption of existing solutions from other spaces.

New Requirements

Now let’s get to the take-aways I had from the workshop’s sessions:

Session 1 / Content Provider and Consumer Perspective:

The sessions participants postulate that we will see the creation of application stores for TV applications similar to how we have experienced this for mobile phones and tablets. People enjoy collecting apps like they collect badges. Right now, the app store domain is dominated by native apps and now Web apps. The reason is that we haven’t got a standard platform for setting up Web app stores with Web apps that work in all browsers on all operating systems. Thus, developers have to re-deploy their app for many environments.

While essentially an orthogonal need to HTML standardisation, this seems to be one of the key issues that keep Web apps back from making big market inroads and W3C may do well in setting up a new WG to define a standard Web app manifest format and JS APIs.

Session 2+3 / Multi-screen TV in the Home Network:

Several technologies of hybrid TV broadcast and set-top-box Web content delivery were being pointed out, including the European HbbTV and the Japanese Hybridcast, the latter of which gave an in-depth demo.

Web purists would probably say that it would be simpler to just deliver all content over the Web and not have to worry about any further technical challenges encountered by having to synchronize content received via two vastly different delivery mechanisms. I personally believe this development is one of business models: we don’t yet know exactly how to earn money from TV content delivered over the Internet, but we do know how to do so with TV content. So, hybrids allow the continuation of existing income streams while allowing the features to be augmented with those people enjoy from the Internet.

Should requirements that emerge from such a use case for HTML5 video be taken seriously? I think they absolutely should. What I see happening is that a new way of using the Web is starting to emerge. The new way is video-focused rather than text-focused. We receive our Web content by watching video programming online – video channels, not Web pages are the core content that we consume in the living room. Video channels are where we start our browsing experience from. Search may still be our first point of call, but it will be search for video content or a video-centric app rather than search for a Web site.

And it will be a matter of many interconnected devices in the house that contribute to the experience: the 5.1 stereos that are spread all over the house and should receive our video’s sound, the different screens in the different areas of our house between which we move around, and remote controls, laptops or tablets that function as remote controls and preview stations and are used to determine our viewing experience and provide a back-channel to the publishers.

We have barely begun to identify how such interconnected devices within a home fit within the server-client-based view of the Web world, and the new Web Sockets functionality. The Home Networking Task Force of the Web and TV IG is looking at the issues and analysing existing protocols and standards that solve this picture. But I have a gnawing feeling that the best solution will be something new that is more Web-specific and fits better with the technology layers of the Web.

Session 4 / Synchronized Metadata:

The TV environment offers many data services, some of which have been legally prescribed. This session analysed TV needs and how they can be satisfied with current HTML5.

Subtitles and closed captioning support are one of the key requirements that have been legally prescribed to allow for equal access of non-native speakers, and blind and vision-impaired users to TV content. After demonstration of some key features defined into the HTML5 track element and the WebVTT format, it was generally accepted that HTML5 is making big progress in this space, in particular that browsers are in the process of implementing support for the track element. A concern still exists for complete coverage of all the CEA-608/708 features in WebVTT.

Further concern was raised for support of audio descriptions and audio translations, in particular since no browser has as yet committed to implementing the HTML5’s media multitrack API with the @mediagroup attribute. In this context I am excited to see first JavaScript polyfills emerge (see captionator.js & mediagroup.js).

Another concern was that many captions are actually delivered as raster images (in particular DVD captions) and how that would work in the Web context. The proposal was to use WebVTT and encode the raster images as data-URIs included in timed cues, then render them by JavaScript as an overlay. This is something to explore further.

Demos were shown using WebVTT to synchronize ads with videos, to display related metadata from a user’s life log with videos, to display thumbnails along a video’s timeline, and to show the rendering of text descriptions through screen readers. General agreement by the panel was that WebVTT offers many opportunities and that this area will continue to need further development and that we will see new capabilities on the Web around metadata that were not previously possible on TV.

Session 5 / Content Format and Codecs: DASH and Codec standards

The introduction of HTTP adaptive streaming into HTML5 was one of the core issues that kept returning in the discussions. This panel focused on MPEG DASH, but also mentioned the need for programmatic implementation of adaptive streaming functionality.

The work around MPEG DASH would require specifications of how to use DASH with WebM and Ogg Theora, as well as a specification of a HTML5 profile for DASH, which would limit the functionality possible in DASH files to the ones needed in a HTML5 video element. One criticism of DASH was its verbosity. Another was its unclear patent position. Panel attendees with included Qualcomm, Apple and Microsoft made very clear that their position is pro a royalty-free use of DASH.

The work around a programmatic implementation for adaptive streaming would require at least a JavaScript API to measure the quality of service of a presented video element and a JavaScript API to feed the video element with chunks of (encrypted) video content on the fly. Interestingly enough, there are existing experiments both around Video metrics and MediaSource extensions, so we can expect some progress in this space, even if these are not yet a strong focus of the HTML WG.

I would personally support the creation of Community Group at the W3C around HTTP adaptive streaming and DASH. I think it would work towards alleviating the perceived patent issues around DASH and allow the right members of the community to participate in preparing a specification for HTML5 without requiring them to become W3C members.

Session 6 / Content Protection and DRM

A core concern of the TV community is around content protection. The requirements in this space seem, however, very confused.

The key assumption here is that Web browsers should support the decoding of DRM-protected content in the HTML5 video element because the video element provides a desirable JavaScript API, accessibility features (the track element), default controls, and the possibility to synchronize multiple media elements. However, at the same time, the video element is part of the core content of a Web page and thus allows direct access to the image content in a canvas etc, so some of its functionality is not desirable.

The picture is further confused by requests for authentication, authorization, encryption, obfuscation, same-origin, secure transmission, secure decryption key delivery, unique content identification and other “content protection” techniques without a clear understanding of what is already possible on the Web and what requirements to content publishers actually have for delivering their content on the Web. This is further complicated by the fact that there are many competing solutions for DRM systems in the market with no clear standard that all browsers could support.

A thorough analysis of the technologies and solutions available in this space as well as an analysis of the needs for HTML5 is required before it becomes clear what solution HTML5 browsers may need to support. There seemed to be agreement in the group, though, that browsers would not need to implement DRM solutions, but rather only hand through the functionality of the platform on which they are running (including the media frameworks and operating system functionalities). How this is supposed to work was, however, unclear.

Session 7 / Web & TV: Additional Device & User Requirements

This was a catch-all session for topics that had not been addressed in other sessions. Among the topics addressed in this group were:

  • Parental Guidance: how to deal with ratings in an internationally inconsistent ratings landscape, how to deliver the ratings with the content, and how to enforce the viewing restrictions
  • Emergency Notifications: how to replicate on the Web the emergency notification functionality of TV by providing text overlays to alert users
  • TV channels: how to detect what channels of programming are available to users

Overall, the workshop was a worthwhile experience. It seems there is a lot of work still ahead for making HTML5 video the best it can be on the Web.

Recent developments around WebVTT

People have been asking me lots of questions about WebVTT (Web Video Text Tracks) recently. Questions about its technical nature such as: are the features included in WebVTT sufficient for broadcast captions including positioning and colors? Questions about its standardisation level: when is the spec officially finished and when will it move from the WHATWG to the W3C? Questions about implementation: are any browsers supporting it yet and how can I make use of it now?

I’m going to answer all of these questions in this post to make it more efficient than answering tweets, emails, and skype and other phone conference requests. It’s about time I do a proper post about it.


I’m starting with the last area, because it is the simplest to answer.

No, no browser has as yet shipped support for the <track> element and therefore there is no support for WebVTT in browsers yet. However, implementations are in progress. For example, Webkit has recently received first patches for the track element, but there is still an open bug for a WebVTT parser. Similarly, Firefox can now parse the track element, but is still working on the element’s actual functionality.

However, you do not have to despair, because there are now a couple of JavaScript polyfill libraries for either just the track element or for video players with track support. You can start using these while you are waiting for the browsers to implement native support for the element and the file format.

Here are some of the libraries that I’ve come across that will support SRT and/or WebVTT (do leave a comment if you come across more):

  • Captionator – a polyfill for track and SRT parsing (WebVTT in the works)
  • js_videosub – a polyfill for track and SRT parsing
  • jscaptions – a polyfill for track and SRT parsing
  • LeanBack player – a video player with track and SRT, SUB, DFXP, and soon full WebVTT parsing support
  • playr – a video player that includes track and WebVTT parsing
  • MediaElementJS – a video player that includes track and SRT parsing
  • Kaltura’s video player – a video player that includes track and SRT parsing

I am actually most excited about the work of Ronny Mennerich from LeanbackPlayer on WebVTT, since he has been the first to really attack full support of cue settings and to discuss with Ian, me and the WHATWG about their meaning. His review notes with visual description of how settings are to be interpreted and his demo will be most useful to authors and other developers.


Before we dig into the technical progress that has been made recently, I want to answer the question of “maturity”.

The WebVTT specification is currently developed at the WHATWG. It is part of the HTML specification there. When development on it started (under its then name WebSRT), it was also part of the HTML5 specification of the W3C. However, there was a concern that HTML5 should be independent of the chosen captioning format and thus WebVTT currently only exists at the WHATWG.

In recent months – and particularly since browser vendors have indicated that they will indeed implement support for WebVTT as their implementation of the <track> element – the question of formal standardization of WebVTT at the W3C has arisen. I’m involved in this as a Google contractor and we’ve put together a proposed charter for a WebVTT Working Group at the W3C.

In the meantime, standardization progresses at the WHATWG productively. Much feedback has recently been brought together by Ian and changes have been applied or at least prepared for a second feature set to be added to WebVTT once the first lot is implemented. I’ve captured the potentially accepted and rejected new features in a wiki page.

Many of the new features are about making the WebVTT format more useful for authoring and data management. The introduction of comments, inline CSS settings and default cue settings will help authors reduce the amount of styling they have to provide. File-wide metadata will help with the exchange of management information in professional captioning scenarios and archives.

But even without these new features, WebVTT already has all the features necessary to support professional captioning requirements. I’ve prepared a draft mapping of CEA-608 captions to WebVTT to demonstrate these capabilities (CEA-608 is the TV captioning standard in the US).

So, overall, WebVTT is in a great state for you to start implementing support for it in caption creation applications and in video players. There’s no need to wait any longer – I don’t expect fundamental changes to be made, but only new features to be added.

New WebVTT Features

This takes us straight to looking at the recently introduced new features.

  • Simpler File Magic:
    Whereas previously the magic file identifier for a WebVTT file was a single line with “WEBVTT FILE”. This has now been changed to a single line with just “WEBVTT”.
  • Cue Bold Span:
    The <b> element has been introduced into WebVTT, thus aligning it somewhat more with SRT and with HTML.
  • CSS Selectors:
    The spec already allowed to use the names of tags, the classes of <c> tags, and the voice annotations of <v> tags as CSS selectors for ::cue. ID selector matching is now also available, where the cue identifier is used.
  • text-decoration support:
    The spec now also supports the CSS text-decoration property for WebVTT cues, allowing functionality such as blinking text and bold.

Further to this, the email identifies the means in which WebVTT is extensible:

  • Header area:
    The WebVTT header area is defined through the “WEBVTT” magic file identifier as a start and two empty lines as an end. It is possible to add into this area file-wide information header information.
  • Cues:
    Cues are defined to start with an optional identifier, and then a start/end time specification with “–>” separator. They end with two empty lines. Cues that contain a “–>” separator but don’t parse as valid start/end time are currently skipped. Such “cues” can be used to contain inline command blocks.
  • Inline in cues:
    Finally, within cues, everything that is within a “tag”, i.e. between “”, and does not parse as one of the defined start or end tags is ignored, so we can use these to hide text. Further, text between such start and end tags is visible even if the tags are ignored, so wen can introduce new markup tags in this way.

Given this background, the following V2 extensions have been discussed:

  • Metadata:
    Enter name-value pairs of metadata into the header area, e.g.

    00:00:15.000 --> 00:00:17.950
    first cue
  • Inline Cue Settings:
    Default cue settings can come in a “cue” of their own, e.g.

    DEFAULTS --> D:vertical A:end
    00:00.000 --> 00:02.000
    This is vertical and end-aligned.
    00:02.500 --> 00:05.000
    As is this.
    DEFAULTS --> A:start
    00:05.500 --> 00:07.000
    This is horizontal and start-aligned.
  • Inline CSS:
    Since CSS is used to format cue text, a means to do this directly in WebVTT without a need for a Web page and external style sheet is helpful and could be done in its own cue, e.g.

      STYLE -->
      ::cue(v[voice=Bob]) { color: green; }
      ::cue(c.narration) { font-style: italic; }
      ::cue(c.narration i) { font-style: normal; }
      00:00.000 --> 00:02.000
      <v Bob>Welcome.
      00:02.500 --> 00:05.000
      <c .narration>To <i>WebVTT</i>.
  • Comments:
    Both, comments within cues and complete cues commented out are possible, e.g.

     COMMENT -->
     00:02.000 --> 00:03.000
     two; this is entirely
     commented out
     00:06.000 --> 00:07.000
     this part of the cue is visible
     <! this part isn't >
     <and neither is this>

Finally, I believe we still need to add the following features:

  • Language tags:
    I’d like to add a language tag that allows to mark up a subpart of cue text as being in a different language. We need this feature for mixed-language cues (in particular where a different font may be necessary for the inline foreign-language text). But more importantly we will need this feature for cues that contain text descriptions rather than captions, such that a speech synthesizer can pick the correct language model to speak the foreign-language text. It was discussed that this could be done with a <lang jp>xxx</lang> type of markup.
  • Roll-up captions:
    When we use timestamp objects and the future text is hidden, then is un-hidden upon reaching its time, we should allow the cue text to scroll up a line when the un-hidden text requires adding a new line. This is the typical way in which TV live captions have been displayed and so users are acquainted with this display style.
  • Inline navigation:
    For chapter tracks the primary use of cues are for navigation. In other formats – in particular in DAISY-books for blind users – there are hierarchical navigation possibilities within media resources. We can use timestamp objects to provide further markers for navigation within cues, but in order to make these available in a hierarchical fashion, we will need a grouping tag. It would be possible to introduce a <nav> tag that can group several timestamp objects for navigation.
  • Default caption width:
    At the moment, the default display size of a caption cue is 100% of the video’s width (height for vertical directions), which can be overruled with the “S” cue setting. I think it should by default rather be the width (height) of the bounding box around all the text inside the cue.

Aside from these changes to WebVTT, there are also some things that can be improved on the <track> element. I personally support the introduction of the source element underneath the track element, because that allows us to provide different caption files for different devices through the @media media queries attribute and it allows support for more than just one default captioning format. This change needs to be made soon so we don’t run into trouble with the currently empty track element.

I further think a oncuelistchange event would be nice as well in cases where the number of tracks is somehow changed – in particular when coming from within a media file.

Other than this, I’m really very happy with the state that we have achieved this far.

WebVTT explained

On Wednesday, I gave a talk at Google about WebVTT, the Web Video Text Track file format that is under development at the WHATWG for solving time-aligned text challenges for video.

I started by explaining all the features that WebVTT supports for captions and subtitles, mentioned how WebVTT would be used for text audio descriptions and navigation/chapters, and explained how it is included into HTML5 markup, such that the browser provides some default rendering for these purposes. I also mentioned the metadata approach that allows any timed content to be included into cues.

The talk slides include a demo of how the <track> element works in the browser. I’ve actually used the Captionator polyfill for HTML5 to make this demo, which was developed by Chris Giffard and is available as open source from GitHub.

The talk was recorded and has been made available as a Google Tech talk with captions and also a separate version with extended audio descriptions.

The slides of the talk are also available (best to choose the black theme).

I’ve also created a full transcript of the described video.

Get the WebVTT specification from the WHATWG Website.

Accessibility to Web video for the Vision-Impaired

In the past week, I was invited to an IBM workshop on audio/text descriptions for video in Japan. Geoff Freed and Trisha O’Connell from WGBH, and Michael Evans from BBC research were the other invited experts to speak about the current state of video accessibility around the world and where things are going in TV/digital TV and the Web.

The two day workshop was very productive. The first day was spent with presentations which were open to the public. A large vision-impaired community attended to understand where technology is going. It was very humbling to be part of an English-spoken workshop in Japan, where much of the audience is blind, but speaks English much better than my average experience with English in Japan. I met many very impressive and passionate people that are creating audio descriptions, adapting NVDA for the Japanese market, advocating to Broadcasters and Government to create more audio descriptions, and perform fundamental research for better tools to create audio descriptions. My own presentation was on “HTML5 Video Descriptions“.

On the second day, we only met with the IBM researchers and focused discussions on two topics:

  1. How to increase the amount of video descriptions
  2. HTML5 specifications for video descriptions

The first topic included concerns about guidelines for description authoring by beginners, how to raise awareness, who to lobby, and what production tools are required. I personally was more interested in the second topic and we moved into a smaller breakout group to focus on these discussions.

HTML5 specifications for video descriptions
Two topics were discussed related to video descriptions: text descriptions and audio descriptions. Text descriptions are descriptions authored as time-aligned text snippets and read out by a screen reader. Audio descriptions are audio recordings either of a human voice or even of a TTS (text-to-speech) synthesis – in either case, they are audio samples.

For a screen reader, the focus was actually largely on NVDA and people were very excited about the availability of this open source tool. There is a concern about how natural-sounding a screen reader can be made and IBM is doing much research there with some amazing results. In user experiment between WGBH and IBM they found that the more natural the voice sounds, the more people comprehend, but between a good screen reader and an actual human voice there is not much difference in the comprehension level. Broadcasters and other high-end producers are unlikely to accept TTS and will prefer the human voice, but for other materials – in particular for the large majority of content on the Web – TTS and screen readers can make a big difference.

An interesting lesson that I learnt was that video descriptions can be improved by 30% (i.e. 30% better comprehension) if we introduce extended descriptions, i.e. descriptions that can pause the main video to allow for a description be read for something that happens in the video, but where there is no obvious pause to read out the description. So, extended descriptions are one of the major challenges to get right.

We then looked at the path that we are currently progressing on in HTML5 with WebSRT, the TimedTrack API, the <track> elements and the new challenges around a multitrack API.

For text descriptions we identified a need for the following:

  • extension marker on cues: often it is very clear to the author of a description cue that there is no time for the cue to be read out in parallel to the main audio and the video needs to be paused. The proposal is for introduction of an extension marker on the cue to pause the video until the screen reader is finished. So, a speech-complete event from the screen reader API needs to be dealt with. To make this reliable, it might make sense to put a max duration on the cue so the video doesn’t end up waiting endlessly in case the screen reader event isn’t fired. The duration would be calculated based on a typical word speaking rate.
  • importance marker on cues: the duration of all text cues being read out by screen readers depends on the speed set-up of the screen reader. So, even when a cue has been created for a given audio break in the video, it may or may not fit into this break. For most cues it is important that they are read out completely before moving on, but for some it’s not. So, an importance maker could be introduced that determines whether a video stops at the end of the cue to allow the screen reader to finish, or whether the screen reader is silenced at that time no matter how far it has gotten.
  • ducking during cues: making the main audio track quieter in relation to the video description for the duration of a cue such as to allow the comprehension of the video description cue is important for comprehension
  • voice hints: an instruction at the beginning of the text description file for what voice to choose such that it won’t collide with e.g. the narrator voice of a video – typically the choice will be for a female voice when the narrator is male and the other way around – this will help initialize the screen reader appropriately
  • speed hints: an indicator at the beginning of a text description toward what word rate was used as the baseline for the timing of the cue durations such that a screen reader can be initialized with this
  • synthesis directives: while not a priority, eventually it will make for better quality synchronized text if it is possible to include some of the typical markers that speech synthesizers use (see e.g. SSML or speech CSS), including markers for speaker change, for emphasis, for pitch change and other prosody. It was, in fact, suggested that the CSS3’s speech module may be sufficient in particular since Opera already implements it.

This means we need to consider extending WebSRT cues with an “extension” marker and an “importance” marker. WebSRT further needs header-type metadata to include a voice and a speed hint for screen readers. The screen reader further needs to work more closely with the browser and exchange speech-complete events and hints for ducking. And finally we may need to allow for CSS3 speech styles on subparts of WebSRT cues, though I believe this latter one is not of high immediate importance.

For audio descriptions we identified a need for:

  • external/in-band descriptions: allowing external or in-band description tracks to be synchronized with the main video. It would be assumed in this case that the timeline of the description track is identical to the main video.
  • extended external descriptions: since it’s impossible to create in-band extended descriptions without changing the timeline of the main video, we can only properly solve the issue of extended audio descriptions through external resources. One idea that we came up with is to use a WebSRT file with links to short audio recordings as external extended audio descriptions. These can then be synchronized with the video and pause the video at the correct time etc through JavaScript. This is probably a sufficient solution for now. It supports both, sighted and vision-impaired users and does not extend the timeline of the original video. As an optimization, we can also do this through a single “virtual” resource that is a concatenation of the individual audio cues and is addressed through the WebSRT file with byte ranges.
  • ducking: making the main audio track quieter in relation to the video description for the duration of a cue such as to allow the comprehension of the video description cue is important for comprehension also with audio files, though it may be more difficult to realize
  • separate loudness control: making it possible for the viewer to separately turn the loudness of an audio description up/down in comparison to the main audio

For audio descriptions, we saw the need for introduction of a multitrack video API and markup to synchronize external audio description tracks with the main video. Extended audio descriptions should be solved through JavaScript and hooking up through the TimedTrack API, so mostly rolling it by hand at this stage. We will see how that develops in future. Ducking and separate loudness controls are equally needed here, but we do need more experiments in this space.

Finally, we discussed general needs to locate accessibility content such as audio descriptions by vision-impaired user:

  • the need for accessible user menus to turn on/off accessibility content
  • the introduction of dedicated and standardized keyboard short-cuts to turn on and manipulate the volume of audio descriptions (and captions)
  • the introduction of user preferences for automatically activating accessibility content; these could even learn from current usage, such that if a user activates descriptions for a video on one Website, the preferences pick this up; different user profiles are already introduced by ISO in “Access for all” and used in websites such as teachersdomain
  • means to generally locate accessibility content on the web, such as fields in search engines and RSS feeds
  • more generally there was a request to have caption on/off and description on/off buttons be introduced into remote controls of machines, which will become prevalent with the increasing amount of modern TV/Internet integrated devices

Overall, the workshop was a great success and I am keen to see more experimentation in this space. I also hope that some of the great work that was shown to us at IBM with extended descriptions and text descriptions will become available – if only as screencasts – so we can all learn from it to make better standards and technology.

WebSRT and HTML5 media accessibility

On 23rd July, Ian Hickson, the HTML5 editor, posted an update to the WHATWG mailing list introducing the first draft of a platform for accessibility for the HTML5 <video> element. The platform provides for captions, subtitles, audio descriptions, chapter markers and similar time-synchronized text both in-band (inside the video resource) and out-of-band (as external text files). Right now, the proposal only regards <video>, but I personally believe the same can be applied to the <audio> element, except we have to be a bit more flexible with the rendering approach. Anyway…

What I want to do here is to summarize what was introduced, together with the improvements that I and some others have proposed in follow-up emails, and list some of the media accessibility needs that we are not yet dealing with.

For those wanting to only selectively read some sections, here is a clickable table of contents of this rather long blog post:


The first and to everyone probably most surprising part is the new file format that is being proposed to contain out-of-band time-synchronized text for video. A new format was necessary after the analysis of all relevant existing formats determined that they were either insufficient or hard to use in a Web environment.

The new format is called WebSRT and is an extension to the existing SRT SubRip format. It is actually also the part of the new specification that I am personally most uncomfortable with. Not that WebSRT is a bad format. It’s just not sufficient yet to provide all the functionality that a good time-synchronized text format for Web media should. Let’s look at some examples.

WebSRT is composed of a sequence of timed text cues (that’s what we’ve decided to call the pieces of text that are active during a certain time interval). Because of its ancestry of SRT, the text cues can optionally be numbered through. The content of the text cues is currently allowed to contain three different types of text: plain text, minimal markup, and anything at all (also called “metadata”).

In its most simple form, a WebSRT file is just an ordinary old SRT file with optional cue numbers and only plain text in cues:

  00:00:15.00 --> 00:00:17.95
  At the left we can see...

  00:00:18.16 --> 00:00:20.08
  At the right we can see the...

  00:00:20.11 --> 00:00:21.96
  ...the head-snarlers

A bit of a more complex example results if we introduce minimal markup:

  00:00:15.00 --> 00:00:17.95 A:start
  Auf der <i>linken</i> Seite sehen wir...

  00:00:18.16 --> 00:00:20.08 A:end
  Auf der <b>rechten</b> Seite sehen wir die....

  00:00:20.11 --> 00:00:21.96 A:end
  <1>...die Enthaupter.

  00:00:21.99 --> 00:00:24.36 A:start
  <2>Alles ist sicher.
  Vollkommen <b>sicher</b>.

and add to this a CSS to provide for some colors and special formatting:

    ::cue { background: rgba(0,0,0,0.5); } 
    ::cue-part(1) { color: red; } 
    ::cue-part(2, b) { font-style: normal; text-decoration: underline; } 

Minimal markup accepts <i>, <b>, <ruby> and a timestamp in <>, providing for italics, bold, and ruby markup as well as karaoke timestamps. Any further styling can be done using the CSS pseudo-elements ::cue and ::cue-part, which accept the features ‘color’, ‘text-shadow’, ‘text-outline’, ‘background’, ‘outline’, and ‘font’.

Note that positioning requires some special notes at the end of the start/end timestamps which can provide for vertical text, line position, text position, size and alignment cue setting. Here is an example with vertically rendered Chinese text, right-aligned at 98% of the video frame:

  00:00:15.00 --> 00:00:17.95 A:start D:vertical L:98%

  00:00:18.16 --> 00:00:20.08 A:start D:vertical L:98%

  00:00:20.11 --> 00:00:21.96 A:start D:vertical L:98%

  00:00:21.99 --> 00:00:24.36 A:start D:vertical L:98%

Finally, WebSRT files can be authored with abstract metadata inside cues, which practically means anything at all. Here’s an example with HTML content:

  00:00:15.00 --> 00:00:17.95 A:start
  <img src="pic1.png"/>Auf der <i>linken</i> Seite sehen wir...

  00:00:18.16 --> 00:00:20.08 A:end
  <img src="pic2.png"/>Auf der <b>rechten</b> Seite sehen wir die....

  00:00:20.11 --> 00:00:21.96 A:end
  <img src="pic3.png"/>...die <a href="http://members.chello.nl/j.kassenaar/

  00:00:21.99 --> 00:00:24.36 A:start
  <img src="pic4.png"/>Alles ist <mark>sicher</mark>.<br/>Vollkommen <b>sicher</b>.

Here is another example with JSON in the cues:

  00:00:00.00 --> 00:00:44.00
    slide: intro.png,
    title: "Really Achieving Your Childhood Dreams" by Randy Pausch, 
             Carnegie Mellon University, Sept 18, 2007

  00:00:44.00 --> 00:01:18.00
    slide: elephant.png,
    title: The elephant in the room...

  00:01:18.00 --> 00:02:05.00
    slide: denial.png,
    title: I'm not in denial...

What I like about WebSRT:

  1. it allows for all sorts of different content in the text cues – plain text is useful for texted audio descriptions, minimal markup is useful for subtitles, captions, karaoke and chapters, and “metadata” is useful for, well, any data.
  2. it can be easily encapsulated into media resources and thus turned into in-band tracks by regarding each cue as a data packet with time stamps.
  3. it is not verbose

Where I think WebSRT still needs improvements:

  1. break with the SRT history: since WebSRT and SRT files are so different, WebSRT should get its own MIME type, e.g. text/websrt, and file extensions, e.g. .wsrt; this will free WebSRT for changes that wouldn’t be possible by trying to keep conformant with SRT
  2. introduce some header fields into WebSRT: the format needs
    • file-wide name-value metadata, such as author, date, copyright, etc
    • language specification for the file as a hint for font selection and speech synthesis
    • a possibility for style sheet association in the file header
    • a means to identify which parser is required for the cues
    • a magic identifier and a version string of the format
  3. allow innerHTML as an additional format in the cues with the CSS pseudo-elements applying to all HTML elements
  4. allow full use of CSS instead of just the restricted features and also use it for positioning instead of the hard to understand positioning hints
  5. on the minimum markup, provide a neutral structuring element such as <span @id @class @lang> to associate specific styles or specific languages with a subpart of the cue

Note that I undertook some experiments with an alternative format that is XML-based and called WMML to gain most of these insights and determine the advantages/disadvantages of a xml-based format. The foremost advantage is that there is no automatism with newlines and displayed new lines, which can make the source text file more readable. The foremost disadvantages are verbosity and that there needs to be a simple encoding step to remove all encapsulating header-type content from around the timed text cues before encoding it into a binary media resource.


Now that we have a timed text format, we need to be able to associate it with a media resource in HTML5. This is what the <track> element was introduced for. It associates the timestamps in the timed text cues with the timeline of the video resource. The browser is then expected to render these during the time interval in which the cues are expected to be active.

Here is an example for how to associate multiple subtitle tracks with a video:

  <video src="california.webm" controls>
    <track label="English" kind="subtitles" src="calif_eng.wsrt" srclang="en">
    <track label="German" kind="subtitles" src="calif_de.wsrt" srclang="de">
    <track label="Chinese" kind="subtitles" src="calif_zh.wsrt" srclang="zh">

In this case, the UA is expected to provide a text menu with a subtitle entry with these three tracks and their label as part of the video controls. Thus, the user can interactively activate one of the tracks.

Here is an example for multiple tracks of different kinds:

  <video src="california.webm" controls>
    <track label="English" kind="subtitles" src="calif_eng.wsrt" srclang="en">
    <track label="German" kind="captions" src="calif_de.wsrt" srclang="de">
    <track label="French" kind="chapter" src="calif_fr.wsrt" srclang="fr">
    <track label="English" kind="metadata" src="calif_meta.wsrt" srclang="en">
    <track label="Chinese" kind="descriptions" src="calif_zh.wsrt" srclang="zh">

In this case, the UA is expected to provide a text menu with a list of track kinds with one entry each for subtitles, captions and descriptions through the controls. The chapter tracks are expected to provide some sort of visual subdivision on the timeline and the metadata tracks are not exposed visually, but are only available through the JavaScript API.

Here are several ideas for improving the <track> specification:

  • <track> is currently only defined for WebSRT resources – it should be made generic and then browsers can compete on the formats for which they provide support. WebSRT could be the baseline format. A @type attribute could be added to hint at the MIME type of the provided resource.
  • <track> needs a means for authors to mark certain tracks as active, others as inactive. This can be overruled by browser settings e.g. on @srclang and by user interaction.
  • karaoke and lyrics are supported by WebSRT, but aren’t in the HTML5 spec as track kinds – they should be added and made visible like subtitles or captions.


This is where we take an extra step and move to a uniform handling of both in-band and out-of-band timed text tracks. Futher, a third type of timed text track has been introduced in the form of a MutableTimedTrack – i.e. one that can be authored and added through JavaScript alone.

The JavaScript API that is exposed for any of these track type is identical. A media element now has this additional IDL interface:

interface HTMLMediaElement : HTMLElement {
  readonly attribute TimedTrack[] tracks;
  MutableTimedTrack addTrack(in DOMString label, in DOMString kind, 
                                 in DOMString language);

A media element thus manages a list of TimedTracks and provides for adding TimedTracks through addTrack().

The timed tracks are associated with a media resource in the following order:

  1. The <track> element children of the media element, in tree order.
  2. Tracks created through the addTrack() method, in the order they were added, oldest first.
  3. In-band timed text tracks, in the order defined by the media resource’s format specification.

The IDL interface of a TimedTrack is as follows:

interface TimedTrack {
  readonly attribute DOMString kind;
  readonly attribute DOMString label;
  readonly attribute DOMString language;
  readonly attribute unsigned short readyState;
           attribute unsigned short mode;
  readonly attribute TimedTrackCueList cues;
  readonly attribute TimedTrackCueList activeCues;
  readonly attribute Function onload;
  readonly attribute Function onerror;
  readonly attribute Function oncuechange;

The first three capture the value of the @kind, @label and @srclang attributes and are provided by the addTrack() function for MutableTimedTracks and exposed from metadata in the binary resource for in-band tracks.

The readyState captures whether the data is available and is one of “not loaded”, “loading”, “loaded”, “failed to load”. Data is only availalbe in “loaded” state.

The mode attribute captures whether the data is activate to be displayed and is one of “disabled”, “hidden” and “showing”. In the “disabled” mode, the UA doesn’t have to download the resource, allowing for some bandwidth management.

The cues and activeCues attributes provide the list of parsed cues for the given track and the subpart thereof that is currently active.

The onload, onerror, and oncuechange functions are event handlers for the load, error and cuechange events of the TimedTrack.

Individual cues expose the following IDL interface:

interface TimedTrackCue {
  readonly attribute TimedTrack track;
  readonly attribute DOMString id;
  readonly attribute float startTime;
  readonly attribute float endTime;
  DOMString getCueAsSource();
  DocumentFragment getCueAsHTML();
  readonly attribute boolean pauseOnExit;
  readonly attribute Function onenter;
  readonly attribute Function onexit;
  readonly attribute DOMString direction;
  readonly attribute boolean snapToLines;
  readonly attribute long linePosition;
  readonly attribute long textPosition;
  readonly attribute long size;
  readonly attribute DOMString alignment;
  readonly attribute DOMString voice;

The @track attribute links the cue to its TimedTrack.

The @id, @startTime, @endTime attributes expose a cue identifier and its associated time interval. The getCueAsSource() and getCueAsHTML() functions provide either an unparsed cue text content or a text content parsed into a HTML DOM subtree.

The @pauseOnExit attribute can be set to true/false and indicates whether at the end of the cue’s time interval the media playback should be paused and wait for user interaction to continue. This is particularly important as we are trying to support extended audio descriptions and extended captions.

The onenter and onexit functions are event handlers for the enter and exit events of the TimedTrackCue.

The @direction, @snapToLines, @linePosition, @textPosition, @size, @alignment and @voice attributes expose WebSRT positioning and semantic markup of the cue.

My only concerns with this part of the specification are:

  • The WebSRT-related attributes in the TimedTrackCue are in conflict with CSS attributes and really should not be introduced into HTML5, since they are WebSRT-specific. They will not exist in other types of in-band or out-of-band timed text tracks. As there is a mapping to do already, why not rely on already available CSS features.
  • There is no API to expose header-specific metadata from timed text tracks into JavaScript. This such as the copyright holder, the creation date and the usage rights of a timed text track would be useful to have available. I would propose to add a list of name-value metadata elements to the TimedTrack API.
  • In addition, I would propose to allow media fragment hyperlinks in a <video> @src attribute to point to the @id of a TimedTextCue, thus defining that the playback position should be moved to the time offset of that TimedTextCue. This is a useful feature and builds on bringing named media fragment URIs and TimedTracks together.


The third part of the timed track framework deals with how to render the timed text cues in a Web page. The rendering rules are explained in the HTML5 rendering section.

I’ve extracted the following rough steps from the rendering algorithm:

  1. All timed tracks of a media resource that are in “showing” mode are rendered together to avoid overlapping text from multiple tracks.
  2. The timed tracks cues that are to be rendered are collected from the active timed tracks and ordered by the timed track order first and by their start time second. Where there are identical start times, the cues are ordered by their end time, earliest first, or by their creation order if all else is identical.
  3. Each cue gets its own CSS box.
  4. The text in the CSS boxes is positioned and formated by interpreting the positioning and formatting instructions of WebSRT that are provided on the cues.
  5. An anonymous inline CSS box is created into which all the cue CSS boxes are wrapped.
  6. The wrapping CSS box gets the dimensions of the video viewport. The cue CSS boxes are positioned so they don’t overlap. The text inside the cue CSS boxes inside the wrapping CSS box is wrapped at the edges if necessary.

To overcome security concerns with this kind of direct rendering of a CSS box into the Web page where text comes potentially from a different and malicious Web site, it is required to have the cues come from the same origin as the Web page.

To allow application of a restricted set of CSS properties to the timed text cues, a set of pseudo-selectors was introduced. This is necessary since all the CSS boxes are anonymous and cannot be addressed from the Web page. The introduced pseudo-selectors are ::cue to address a complete cue CSS box, and ::cue-part to address a subpart of a cue CSS box based on a set of identifiers provided by WebSRT.

I have several issues with this approach:

  • I believe that it is not a good idea to only restrict rendering to same-origin files. This will disallow the use of external captioning services (or even just a separate caption server of the same company) to link to for providing the captions to a video. Henri Sivonen proposed a means to overcome this by parsing every cue basically as its own HTML document (well, the body of a document) and then rendering these in iFrame-manner into the Web page. This would overcome the same-origin restriction. It would also allow to do away with the new ::cue CSS selectors, thus simplifying the solution.
  • In general I am concerned about how tightly the rendering is tied to WebSRT. Step 4 should not be in the HTML5 specification, but only apply to WebSRT. Every external format should provide its own mapping to CSS. As it is specified right now, other formats, such as e.g. 3GPP in MPEG-4 or Kate in Ogg, are required to map their format and positioning information to WebSRT instructions. These are then converted again using the WebSRT to CSS mapping rules. That seems overkill.
  • I also find step 6 very limiting, since only the video viewport is regarded as a potential rendering area – this is also the reason why there is no rendering for audio elements. Instead, it would make a lot more sense if a CSS box was provided by the HTML page – the default being the video viewport, but it could be changed to any area on screen. This would allow to render music lyrics under or above an audio element, or render captions below a video element to avoid any overlap at all.


We’ve made huge progress on accessibility features for HTML5 media elements with the specifications that Ian proposed. I think we can move it to a flexible and feature-rich framework as the improvements that Henri, myself and others have proposed are included.

This will meet most of the requirements that the W3C HTML Accessibility Task Force has collected for media elements where the requirements relate to accessibility functionality provided through alternative text resources.

However, we are not solving any of the accessibility needs that relate to alternative audio-visual tracks and resources. In particular there is no solution yet to deal with multi-track audio or video files that have e.g. sign language or audio description tracks in them – not to speak of the issues that can be introduced through dealing with separate media resources from several sites that need to be played back in sync. This latter may be a challenge for future versions of HTML5, since needs for such synchoronisation of multiple resources have to be explored further.

In a first instance, we will require an API to expose in-band tracks, a means to control their activation interactively in a UI, and a description of how they should be rendered. E.g. should a sign language track be rendered as pciture-in-picture? Clear audio and Sign translation are the two key accessibility needs that can be satisfied with such a multi-track solution.

Finally, another key requirement area for media accessibility is described in a section called “Content Navigation by Content Structure”. This describes the need for vision-impaired users to be able to navigate through a media resource based on semantic markup – think of it as similar to a navigation through a book by book chapters and paragraphs. The introduction of chapter markers goes some way towards satisfying this need, but chapter markers tend to address only big time intervals in a video and don’t let you navigate on a different level to subchapters and paragraphs. It is possible to provide that navigation through providing several chapter tracks at different resolution levels, but then they are not linked together and navigation cannot easily swap between resolution levels.

An alternative might be to include different resolution levels inside a single chapter track and somehow control the UI to manage them as different resolutions. This would only require an additional attribute on text cues and could be useful to other types of text tracks, too. For example, captions could be navigated based on scenes, shots, coversations, or individual captions. Some experimentation will be required here before we can introduce a sensible extension to the given media accessibility framework.

“HTML5 Audio And Video Accessibility, Internationalisation And Usability” talk at Mozilla Summit

For 2 months now, I have been quietly working along on a new Mozilla contract that I received to continue working on HTML5 media accessibility. Thanks Mozilla!

Lots has been happening – the W3C HTML5 accessibility task force published a requirements document, the Media Text Associations proposal made it into the HTML5 draft as a <track> element, and there are discussions about the advantages and disadvantages of the new WebSRT caption format that Ian Hickson created in the WHATWG HTML5 draft.

In attending the Mozilla Summit last week, I had a chance to present the current state of development of HTML5 media accessibility and some of the ongoing work. I focused on the following four current activities on the technical side of things, which are key to satisfying many of the collected media accessibility requirements:

  1. Multitrack Video Support
  2. External Text Tracks Markup in HTML5
  3. External Text Track File Format
  4. Direct Access to Media Fragments

The first three now already have first drafts in the HTML5 specification, though the details still need to be improved and an external text track file format agreed on. The last has had a major push ahead with the Media Fragments WG publishing a Last Call Working Draft. So, on the specification side of things, major progress has been made. On the implementation – even on the example implementation – side of things, we still fall down badly. This is where my focus will lie in the next few months.

Follow this link to read through my slides from the Mozilla 2010 summit.

Media Fragment URI Specification in Last Call WD

After two years of effort, the W3C Media Fragment WG has now created a Last Call Working Draft document. This means that the working group is fairly confident that they have addressed all the required issues for media fragment URIs and their implementation on HTTP and is asking for outside experts and groups for input. This is the time for you to get active and proof-read the specification thoroughly and feed back all the concerns that you have and all the things you do not understand!

The media fragment (MF) URI specification specifies two types of MF URIs: those created with a URI fragment (“#”), e.g. video.ogv#t=10,20 and those with a URI query (“?”), e.g. video.ogv?t=10,20. There is a fundamental difference between the two that needs to be appreciated: with a URI fragment you can specify a subpart of a resource, e.g. a subpart of a video, while with a URI query you will refer to a different resource, i.e. a “new” video. This is an important difference to understand for media fragments, because only some things that we want to achieve with media fragments can be achieved with “#”, while others can only be achieved by transforming the resource into a different new bitstream.

This all sounds very abstract, so let me give you an example. Say you want to retrieve a video without its audio track. Say you’d rather not download the audio track data, since you want to save on bandwidth. So, you are only interested to get the video data. The URI that you may want to use is video.ogv#track=video. This means that you don’t want to change the video resource, but you only want to see the video. The user agent (UA) has two options to resolve such a URI: it can either map that request to byte ranges and just retrieve those – or it can download the full resource and ignore the data it has not been requested to display.

Since we do not want the extra bytes of the audio track to be retrieved, we would hope the UA can do the byte range requests. However, most Web video formats will interleave the different tracks of a media resource in time such that a video track will results in a gazillion of smaller byte ranges. This makes it impractical to retrieve just the video through a “#” media fragment. Thus, if we really want this functionality, we have to make the server more intelligent and allow creation of a new resource from the existing one which doesn’t contain the audio. Then, the server, upon receiving a request such as video.ogv#track=video can redirect that to video.ogv?track=video and actually serve a new resource that satisfies the needs.

This is in fact exactly what was implemented in a recently published Firefox Plugin written by Jakub Sendor – also described in his presentation “Media Fragment Firefox plugin”.

Media Fragment URIs are defined for four dimensions:

  • temporal fragments
  • spatial fragments
  • track fragments
  • named fragments

The temporal dimension, while not accompanied with another dimension, can be easily mapped to byte ranges, since all Web media formats interleave their tracks in time and thus create the simple relationship between time and bytes.

The spatial dimension is a very complicated beast. If you address a rectangular image region out of a video, you might want just the bytes related to that image region. That’s almost impossible since pixels are encoded both aggregated across the frame and across time. Also, actually removing the context, i.e. the image data outside the region of interest may not be what you want – you may only want to focus in on the region of interest. Thus, the proposal for what to do in the spatial dimension is to simply retrieve all the data and have the UA deal with the display of the focused region, e.g. putting a dark overlay over the regions outside the region of interest.

The track dimension is similarly complicated and here it was decided that a redirect to a URI query would be in order in the demo Firefox plugin. Since this requires an intelligent server – which is available through the Ninsuna demo server that was implemented by Davy Van Deursen, another member of the MF WG – the Firefox plugin makes use of that. If the UA doesn’t have such an intelligent server available, it may again be most useful to only blend out the non-requested data on the UA similar to the spatial dimension.

The named dimension is still a largely undefined beast. It is clear that addressing a named dimension cannot be done together with the other dimensions, since a named dimension can represent any of the other dimensions above, and even a combination of them. Thus, resolving a named dimension requires an understanding of either the UA or the server what the name maps to. If, for example, a track has a name in a media resource and that name is stored in the media header and the UA already has a copy of all the media headers, it can resolve the name to the track that is being requested and take adequate action.

But enough explaining – I have made a screencast of the Firefox plugin in action for all these dimensions, which explains things a lot more concisely than word will ever be able to – enjoy:

And do not forget to proofread the specification and send feedback to public-media-fragment@w3.org.