Untested Areas of WebVTT Spec

[danae404]

In this issue we will keep a running list of areas of the WebVTT spec that are not currently tested by WPT. We are retroactively listing any areas that we have added tests for since the beginning of the investigation, for the purpose of keeping track of progress. If an item on the list is checked, it means that we have added test coverage for it.

• ::cue(|) (This is somewhat tested in root_namespace.html, but it gets overridden by another selector, so we can't be certain that it is functional)
• ::cue([lang=]) and ::cue(lang) selectors
• Media controls showing