requestAnimationFrame是实现如“最优帧速率”、“选择绘制下一帧的最佳时机”的功能，它的兼容性写法：
var rAF = window.requestAnimationFrame || window.webkitRequestAnimationFrame || window.mozRequestAnimationFrame || window.oRequestAnimationFrame || window.msRequestAnimationFrame || function (callback) { window.setTimeout(callback, 1000 / 60); };
原文：http://www.cnblogs.com/liuyinlei/p/6761036.html