Changes

* However, exposing them as direct subelements is probably not the way to go for two reasons:

1) The general expectation is that the DOM tree is the parse tree of one HTTP resource--that is, there's one Document object per HTTP resource.

2) For security reasons, allowing a video file cause node insertion into an HTML DOM tree seems like a potentially dangerous thing.

* If a DOM tree representation is natural for the captioning format, it should probably be available under a separate Document object like iframe content.