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window.requestAnimationFrame()

Pushes support back to at least IE6.

/**
 * requestAnimationFrame() polyfill
 * By Erik Möller. Fixes from Paul Irish and Tino Zijdel.
 * @link http://paulirish.com/2011/requestanimationframe-for-smart-animating/
 * @link http://my.opera.com/emoller/blog/2011/12/20/requestanimationframe-for-smart-er-animating
 * @license MIT
 */
(function() {
	var lastTime = 0;
	var vendors = ['ms', 'moz', 'webkit', 'o'];
	for(var x = 0; x < vendors.length && !window.requestAnimationFrame; ++x) {
		window.requestAnimationFrame = window[vendors[x]+'RequestAnimationFrame'];
		window.cancelAnimationFrame = window[vendors[x]+'CancelAnimationFrame'] ||
		                              window[vendors[x]+'CancelRequestAnimationFrame'];
	}

	if (!window.requestAnimationFrame) {
		window.requestAnimationFrame = function(callback, element) {
			var currTime = new Date().getTime();
			var timeToCall = Math.max(0, 16 - (currTime - lastTime));
			var id = window.setTimeout(function() { callback(currTime + timeToCall); },
				timeToCall);
			lastTime = currTime + timeToCall;
			return id;
		};
	}

	if (!window.cancelAnimationFrame) {
		window.cancelAnimationFrame = function(id) {
			clearTimeout(id);
		};
	}
}());