[whatwg] How to expose caption tracks without TextTrackCues

Hi all,

In the Inband Text Tracks Community Group we've recently had a
discussion about a proposal by HbbTV. I'd like to bring it up here to
get some opinions on how to resolve the issue.

(The discussion thread is at
, but let me summarize it here, because it's a bit spread out.)

The proposed use case is as follows:
* there are MPEG-2 files that have an audio, a video and several caption tracks
* the caption tracks are not in WebVTT format but in formats that
existing Digital TV receivers are already capable of decoding and
displaying (e.g. CEA708, DVB-T, DVB-S, TTML)
* there is no intention to standardize a TextTrackCue format for those
other formats (statements are: there are too many formats to deal
with, a set-top-box won't need access to cues)

The request was to expose such caption tracks as textTracks:
interface HTMLMediaElement : HTMLElement {
  readonly attribute TextTrackList textTracks;

Then, the TextTrack interface would list them as a kind="captions",
but without any cues, since they're not exposed. This then allows
turning the caption tracks on/off via JavaScript. However, for
JavaScript it is indistinguishable from a text track that has no
captions. So the suggestion was to introduce a new kind="UARendered".

My suggestion was to instead treat such tracks as burnt-in video
tracks (by combination with the main video track):
interface HTMLMediaElement : HTMLElement {

readonly attribute VideoTrackList videoTracks;

Using the VideoTrack interface it would list them as a kind="captions"
and would thus also be able to be activated by JavaScript. The
downside would that if you have N video tracks and m caption tracks in
the media file, you'd have to expose NxM videoTracks in the interface.

So, given this, should we introduce a kind="UARendered" or expose such
tracks a videoTracks or is there another solution that we're


Received on Sunday, 12 October 2014 09:46:32 UTC