Looking at Tor suggestions of how to watch youtube video, one has observed that the suggestion to use HTML5 to watch youtube video is quietly disappeared. Is some type of new leak, problem, and/or anonymity/security compromise with respect to HTML5 have been discovered?
The download page had a warning, that you should visit https://www.youtube.com/html5 and enable HTML5 support. So you do not need flash to watch YouTube videos. However right now YouTube works without going first to this site. So this warning as removed with commit
Now you can directly go to https://youtube.com and watch a video without Flash.