HTML5 Youtube no workee

HTML5 video works perfectly with most sites using Torbutton + Firefox 4, for example http://www.html5video.org/. However, the Youtube HTML5 Beta programme does not. I have no idea why: I’ve flipped just about every setting there is in Torbutton, but no dice. Strangely, Youtube videos embedded into other sites work, and youtube.com also does when not using Torbutton, but setting the proxy manually.

Looking at the source code of any youtube page loaded via Torbutton, it seems like Youtube is having trouble detecting either the html5 opt-in or browser capabilities and presents the flash version instead. But as I said, no idea why. Javascript and cookies are enabled.

Trac:
Username: pstumpf