Having tested the PS Vita browser myself, I can only say that it sucks. It's just as bad as the Nintendo Wii U browser (it's really just the bare basic Webkit with only a few extensions, if any).
I honestly don't even know why anyone would want to develop an HTML5 game on PS Mobile though.
Monodevelop is a great IDE and Sony put some serious work into the documentation of their libraries. The included examples basically make your game for you.
The only downside to C# is the fact that doing Functional (Reactive) Programming is tough as nail*. But then, that's not something HTML5 developers would be used to anyways.
*I'm still struggling to re-code any of the included examples to a purely functional approach, as I'm really opposed to the anti-modular/anti-parallel concept of classes