getComputedStyle.js

if (!window.getComputedStyle) {

  window.getComputedStyle = (function() {

    var // partially grabbed from jQuery and Dean's hack
      notpixel = /^(?:[+-]?(?:\d*\.|)\d+(?:[eE][+-]?\d+|))(?!px)[a-z%]+$/,
      position = /^(top|right|bottom|left)$/,
      re = /\-([a-z])/g,
      place = function (match, $1) {
        return $1.toUpperCase();
      }
    ;

    function ComputedStyle(_) {
      this._ = _;
    }

    ComputedStyle.prototype.getPropertyValue = function (name) {
      var
        el = this._,
        style = el.style,
        currentStyle = el.currentStyle,
        runtimeStyle = el.runtimeStyle,
        result,
        left,
        rtLeft
      ;
      name = (name === 'float' ? 'style-float' : name).replace(re, place);
      result = currentStyle ? currentStyle[name] : style[name];
      if (notpixel.test(result) && !position.test(name)) {
        left = style.left;
        rtLeft = runtimeStyle && runtimeStyle.left;
        if (rtLeft) {
          runtimeStyle.left = currentStyle.left;
        }
        style.left = name === 'fontSize' ? '1em' : result;
        result = style.pixelLeft + 'px';
        style.left = left;
        if (rtLeft) {
          runtimeStyle.left = rtLeft;
        }
      }
      return result == null ?
        result : ((result + '') || 'auto');
    };

    // unsupported
    function PseudoComputedStyle() {}
    PseudoComputedStyle.prototype.getPropertyValue = function () {
      return null;
    };

    return function (el, pseudo) {
      return pseudo ?
        new PseudoComputedStyle(el) :
        new ComputedStyle(el);
    };

  })();

}