yet another idea for the video tag.

The video tag specification allows comments to be made, that then show up as subtitles on the video.  Here is another idea.  Lets suppose that I'm a teacher of some subject, and I want to add comments to someone elses video (maybe a students) and on top of that, I want to show an arrow on the video, that points to a specific item on the picture.  Rather than edit the video with some video-editing software, it would be great if I could specify an x-coordinate, a y-coordinate, and an arrow direction that would overlay the video at a certain position in the video, for a certain time.  -- Thanks,Gideon