Hi Chris,

MPEG4 already has in-band text tracks, which at least Safari, but
probably also IE are going to decode into TextTrack objects.
WebM is going to have in-band text tracks with WebVTT. I'm pretty sure
that it will be supported at least in Chrome.

I think it makes sense for Ogg to at least have a mapping. Then you
can make the decision for in-band support in Firefox both for Ogg and
WebM at a later stage.

Cheers,
Silvia.