There's no question that HTML5 is good for more than just video, but if anything will put that misconception to rest, it's Quake II running in a browser. Apparently built by a trio of Google developers in their spare time, the Quake II GWT port uses a HTML5 canvas and WebGL for graphics acceleration (also seen demoed on the N900), which seem to get the job done remarkably well -- although it's still a long ways from running on certain non-Flash-equipped devices. Then again, this project was revealed on April 1st, but quite a few folks have already reported success in getting it up and running -- so, unless there's a global conspiracy to dash our hopes and dreams (always a possibility), it seems to be the real deal.
