![Screen Shot 2013-04-19 at 10 25 57](https://f.cloud.github.com/assets/1059272/400258/9b11acea-a898-11e2-9b01-cfef19e1fc0a.png)
The fps is unstable even in HelloWorld with this PR. It is important to keep it stable when reducing fps. 

I think skipping frame with requestAnimationFrame can not meet this requirement. We had tested skipping frame by toggling with requestAnimationFrame, and its fps is more than 40.