HTML5 videos use HTTP protocol, so it is easy to download them.
Safari and Chrome support HTML5 h.264 videos. h.264 is patent-encumbered, so Firefox decided not to support it. But Firefox supports HTML5 OGG Theora videos.
I hope that HTML5 h.264 videos that are used on YouTube and Vimeo will not become widespread, and that open standards will be used instead of proprietary technologies.
To find a URL of the HTML5 video, you can try to view the source of the page. You can also try to use freeware
URL Snooper. Then just download it with your browser or any download manager.
You can also download such videos with software like
Replay Media Catcher. The following video shows how to set up
Replay Media Catcher to download HTML5 OGG Theora videos :