A user named sKunKeNsTeiN has posted a video on youtube comparing available browsers for N9.He has done HTML5 test for PR1.2 browser and the official version of Firefox and Opera mobile browsers.
The scores are,
PR 1.2 default browser – 271 + 14 Bonus points
Firefox – 315 + 9 Bonus points
Opera mobile browser- 245
Firefox seems to be the king of browsers as far as HTML5 compatibility is concerned.