I totally agree with tulamide!
I remember ages back when I first sent my game to someone after knowingly getting full 60fps in Chrome and not much going on in the game, and they ended up getting like 15fps in Chrome (their machine could get 30fps out of a game like Left4Dead and 50-60fps out of Terraria).
I was so sad, til I realised that shrinking Chrome's window size boosted the framerate (it had to be shrunk so the game was like 320x300 to get 60fps lol), which was weird since Terraria runs nicely on their machine at 1360x768.
But yeah I open my project on Chrome these days on my mid-range computer and get 60fps, but sometimes get SFX not playing, whereas I load it on Firefox, and I get choppy 20fps for some reason, but the music and SFX play fine... I don't even know why lol. I do like how there's no incompatibility at ALL with how things display as far as I'm aware (I think the HSL shader was being weird depending on browser but I think that got sorted out), so sprites and the actual events run great on each browser. Lol but when it comes to wrappers, you realise the Button object don't work, some physics functions aren't working (I'm looking at you CocoonJS <img src="smileys/smiley22.gif" border="0" align="middle" /> ).
I'm not super involved with the mechanics of how HTML5 works and whatnot, but I'll give my opinion anyway:
HTML5 is young, but it's rapidly developing. Seems that all browsers adopted it, all the new consoles are adopting it, heck, Unreal even supports it like Kyatric posted. All I'm going to do, is wait patiently...
*1 year later, HTML6 comes out, not compatible with HTML5*