Aside from doomlaser's answer, have you seriously seen a single demo in HTML5? As I've not seen one yet that doesn't stutter or crash. Even in Chrome with v8.

HTML5 is like 5 years behind Flash in terms of performance.

Also writing in HTML5 at the moment is like being given the total basics. Canvas, as an API, sucks. It'll get better, more libraries will come out, but god-damn it feels like moving backwards to take a step forwards.