"partial interface TextTrack {
  attribute DOMString kind;
  attribute DOMString language;
}"

The TextTrack interface in the html spec has readonly attributes of the same name. I think the idea is to use specific constructors as the only way to pass these in, or have them created by the parser for in-band text tracks and <track> elements.

Do we need this interface? If it's muxed with the parent media element's source stream, the in-band text track support should cover it, and if it's external, the media.AddTextTrack and TextTrack.addCue methods should be sufficient to implement dynamic subtitles with minimal complexity.