TextTrack API changes

Hi all,

Recently, I cherry-picked some changes to the TextTrack API from the WHATWG
repository into the HTML5 specification.

In particular, I am referring to these patches:

** Split TextTrackCue into an abstract TextTrackCue interface and a
WebVTT-specific interface WebVTTCue. Makes it easier to use TextTrack with
other file formats.
https://github.com/w3c/html/commit/586ae3996fdce5d9f71cbe57a08759fce7b26d8f
WHATWG: 98cdbf20015b11ae7febc581280c3ce02dcd800e (7742)

** Split more WebVTT-specific things into the WebVTT spec. This also makes
some normative changes to HTML for handling non-WebVTT cue types, but that
shouldn't affect any existing implementations.
https://github.com/w3c/html/bdae138d123ddb73586eb8d7f39761ec93e3aa28
WHATWG: 0776094323b3f44cbf88eb9f023f4b12c3a6b6a9 (7748)

The aim of these patches was two-fold:

Firstly, they provide for a cleaner cut between the WebVTT specification
and the HTML specification. This was in preparation for a removal of the
WebVTT text from the source file from which the HTML specification is
created such that the WebVTT specification can now be edited separately
(see https://dvcs.w3.org/hg/text-tracks/raw-file/default/webvtt/webvtt.html
).

Secondly, these changes make the TextTrack API abstract and thus more
easily extensible to other file formats such as TTML.

The downside of the changes is that TextTrackCue is now an abstract
interface without a constructor (instead, the WebVTT spec provides the
WebVTTCue constructor). This breaks existing implementations of the
TextTrackCue interface in webkit-based browsers (including blink) and in
presto. IIUC, Mozilla and IE are not supporting TextTrackCue yet. Also,
analysis on the webdevdata collection suggests that the TextTrackCue
constructor is not used much on the Web yet, so this is still a good time
to break the interface (see
http://lists.w3.org/Archives/Public/public-texttracks/2013Apr/0006.html).

While I have right now only applied these changes to HTML5.1, I am
considering applying them to HTML5.0 as well if presto, webkit and blink
decide to change their implementation and gecko and trident decide to
support the new specification. I am looking for advice on such a move.

Best Regards,
Silvia.