From a <video/> element, you can play with the pixels of each frame. For example, you can replace each green pixels by a transparent one.

Here is little demo (Firefox 3.1 needed): http://www.mozbox.org/pub/green/.

42 lines of Javascript, powerful :)

If you don't have Firefox 3.1, see the Chris' screencast: http://www.0xdeadbeef.com/weblog/?p....