Plus, what do you mean by "most browsers"? Are you referring to IE? That's only included in IE9 and IE10, which is like what - 10% of the browser market? You can't include IE6-IE8 in that. So you're only getting about 10% extra market when you're thinking about creating a 2D web game as opposed to a WebGL game, but you lose a 10x performance improvement.
As for mobile browsers - do 2D web games get good performance on mobile? Is anyone even thinking about playing 2D web games on their mobile devices? I assume that even if they work okay, that's still only in the "high-end/latest" mobile devices, which is just a part of the whole smartphone/tablet market.
EDIT: Found it. They show the Microsoft fish demo at 1000 fish with canvas 2D, and then a WebGL version of it with 10,000 fish: