Define how styling works

[hober]

You can use ::cue and ::cue-region from the embedding page (right?), but how can you use CSS from within the cue fragment itself? What sorts of CSS can you use?

Note that in WebVTT today, ::cue has a limited set of styling that can be applied to it. Should that subset be the subset going forward?

In the WebKit prototype, only inline style attributes work. This is not great; we should support the stye element as well. Are there any issues speccing that? How does style get scoped?