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Resolving TextTrackCue issues

From: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Date: Sat, 31 Aug 2013 17:26:51 +1000
Message-ID: <CAHp8n2nMwkDNzECSJ5kyhRHi7WXrXJA25RwBhqb=p1fCGQXkMg@mail.gmail.com>
To: public-html <public-html@w3.org>
Hi all,

Recent changes to the TextTrackCue interface had led to a fork with
the WHATWG spec [1] when resolving bug 21851 [2].

This caused extensive discussion on blink-dev [3] when an intent to
implement was proposed.

In the W3C WG we recognize the need for a generic cue interface type
with a constructor and a text attribute. It allows browsers to expose
cues in text tracks of video or audio files for which browsers don't
intend to implement parsers. It also allows JavaScript developers to
create time-synchronized data for media elements in any format they

The discussion on blink-dev exposed that the currently specified
solution of bug 21851 [2] in the HTML5 spec is flawed in several ways:

(1) TextTrackCue objects that are not fully abstract create hard to
debug issues of backwards compatibility due to existing code that
assumes "new TextTrackCue()" constructs a cue with VTT semantics;
(2) in order to transition old TextTrackCue interface usage to "new
VTTCue()", it is better to remove the existing TextTrackCue
constructor causing hard failure (easily recognizable) instead of soft
failure (more difficult to recognize);
(3) the abstract TextTrackCue interface of the WHATWG is desirable for
extensibility to non-text-based cue interfaces of the future;
(4) the interface fork between the WHATWG and W3C spec should be removed.

An alternative resolution to bug 21851 [2] has previously been
proposed and discussed: create a new interface that has the text
attribute and the constructor and inherits from the abstract

This will result in the following interfaces:

interface TextTrackCue : EventTarget {
  readonly attribute TextTrack? track;

           attribute DOMString id;
           attribute double startTime;
           attribute double endTime;
           attribute boolean pauseOnExit;

           attribute EventHandler onenter;
           attribute EventHandler onexit;

[Constructor(double startTime, double endTime, DOMString text)]
interface GenericCue : TextTrackCue {
           attribute DOMString text;

Whether VTTCue will inherit from GenericCue or from TextTrackCue will
be resolved in the TextTrack CG once this change has been applied to
the HTML5 spec.

It is my understanding that this proposed change resolves all the
above listed issues. I will therefore apply these changes next week
unless there are any further concerns.

Silvia (as HTML spec editor).

[1] https://www.w3.org/Bugs/Public/show_bug.cgi?id=22903
[2] https://www.w3.org/Bugs/Public/show_bug.cgi?id=21851
[3] https://groups.google.com/a/chromium.org/d/msg/blink-dev/-VHGnuNNUxM/Yibbv2TgDoYJ
Received on Saturday, 31 August 2013 07:27:38 UTC

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