Great news today! Google Chrome 15 has been released, and by the looks of things it has a hardware accelerated canvas. This means it uses your graphics card to draw the games, which makes them run super fast - a smooth 60 fps even in fullscreen.
This means the "big three" browsers all have hardware acceleration and should all have excellent performance now: IE9+, Firefox 7+ and Chrome 15+. Chrome always updates to the latest version so within a couple of weeks that should be another 20% of the web using hardware acceleration.
By my reckoning that will make 50% of all web surfers with super fast HTML5 enabled browsers. It's only a matter of time before it's everyone :)