Re: a :time pseudo-class for media elements (and examples on how it would be useful and generic)

I don’t know if other use-cases could benefit from this (maybe there are) but, at least, solving the subtitle (video captionning) problem is probably better done using the existing <track /> element. See http://blogs.msdn.com/b/ie/archive/2011/10/12/html5-video-captioning.aspx for more info.

Another idea to solve this broader kind of problem more efficiently than using a pseudo-class would be to be able to specify a timesource for animations that’s different from the computer clock.
  .video-related {
      animation-time-source: url(#videoID);
  }
Best regards,
François