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Should changing <track language> and company affect the TextTrack object? The spec currently says yes. There are no known use cases as far as I'm aware. It increases complexity a bit. On the other hand, cases where we have mutable attributes where mutating the attribute has no effect on the underlying data have resulted in some author confusion before (notably <script src>). If we changed this we'd also have to make the TextTrack object only get created on demand, since otherwise you could never set the language since the immutable TextTrack would be created before you had a chance to set the element's attributes. This would also break the way things work today when you move a <track> between <video> elements. I think we're probably better off not changing any of this at this late stage. This bug was split from bug 24563.
Yeah, the problem here is that there's no obvious time to create an immutable TextTrack object. On first access by a script won't be great, because then just printing it for debugging will have side effects...
Yeah, I think if we change this the only way to do it really would be to create the TextTrack object during media resource loading (or when the <track> is inserted, whichever happens last), and then create a new TextTrack if it's reused. I've no idea if that would be Web-compatible. It's probably not worth finding out.
Do you mean so that the out-of-band tracks become available at the same time as in-band tracks? That doesn't sound too bad. There's a Blink use counter for any VTTCue usage which gives some idea of risk: http://www.chromestatus.com/metrics/feature/timeline/popularity/409 Any thoughts, Silvia or Simon?
FWIW: I don't think it makes sense to change kind, srclang, label, id or default of a track.
I don't like the idea of making <track>'s TextTrack not be available immediately. It seems like it makes it harder for authors to do things with it in script if you have to wait for the video's loadedmetadata event. For instance, a script might want to change the .mode of the TextTracks.
Given the discussion here I think this should be left as-is. I'm marking this WONTFIX. Please reopen if you think that's the wrong decision.