[detour]

I'm dabbling in this a bit, currently designing a game as a hobby project. There's a number of frameworks available that can ease development though none are perfect.

LimeJs (http://www.limejs.com/) - seems the most mature. Its built upon Google's Closure library. I personally found it a bit cumbersome, though certainly powerful. Optimal performance is obtained by using the Closure compiler.

CraftyJs (http://craftyjs.com/) - was the easiest to get something up and running quickly. If you've ever played around with Unity3D, it basically follows the GameObject model wrapped in a jQuery like syntax; which feels very natural. Performance was decent but I've run into bugs as the project is fairly new.

Cocos2D Js (http://cocos2d-javascript.org/) and EaselJs (http://www.easeljs.com/) - are probably good options but I preferred the other two.

There's also a few vaporware projects that would be amazing if they would ever release, namely RocketPack (http://rocketpack.fi/) and GameClosure (http://gameclosure.com/)