I am currently converting a flash site to html5 and would like some clarification on the abilities / limitations of html5 and its support with alpha channels.

1. Do the video or canvas tag elements even allow the ability to work with alpha channels ?

If yes....
2. What is the best video format / codec with alpha channels for support with html5?

While researching on information for .h264 with alpha channels i came across using offscreen HTML5 Canvas to basically composite 2 separate clips ( h.264 and alpha ) together.

This seems like a huge workaround, taxing and would be computationally heavy.

Any comments or input on these issues would be much appreciated.