Wow, a year ago the situation was quite "depressing". Most YouTube videos only worked with Flash (or not at all for boycotters
) and most browsers sucked at HTML5.
A lot has changed since then!
Most videos work perfectly on Opera 11.60. I only had a sound problem. Until today: I updated the GStreamer framework/plugins and all multimedia libraries (libogg, libtheora, etc.) and now I have sound too. Playback is good enough, even though the CPU usage is very high due to disable hardware acceleration on my system.