I want to explore how Mozilla is currently dealing with the <track> element for HTML5.
According to the Mozilla documentation it is currently not supported. According to the Mozilla bug tracker, we at Seneca College are working on it. There is nothing in the documentation for WebVTT.
The <video> tag is supported. Here is the bug tracker for it.
The DOM media element is here in the documentation.
So now I'm looking at the source code. There is a nice post for the directory structure.
This looks like what deals with the media element.
Anyway, it will take a lot of time to figure out where WebVTT and <track> will plug in.