Chrome and Firefox are the best two browsers for HTML5 games. IE9 is pretty good too, but is let down by a lack of the latest features (e.g. no advanced audio support, no webcam support, no fullscreen API...) and also doesn't support WebGL, so it tends to be a bit slower. Opera isn't very good, not even Opera 12, because it has hardware acceleration off by default, so it's usually very slow. Safari for Windows is the same level as Opera 12. Safari for Mac is OK though.
Hmm, might write up a tutorial with this info.