function _getPropValue (element, propName) {
    var propValue = '';
    if (element.currentStyle) { //IE
      propValue = element.currentStyle[propName];
    } else if (document.defaultView && document.defaultView.getComputedStyle) { //Others
      propValue = document.defaultView.getComputedStyle(element, null).getPropertyValue(propName);
    }

    //Prevent exception in IE
    if (propValue && propValue.toLowerCase) {
      return propValue.toLowerCase();
    } else {
      return propValue;
    }
  }
Get an element CSS property on the page
Thanks to JavaScript Kit: http://www.javascriptkit.com/dhtmltutors/dhtmlcascade4.shtml

@api private
@method _getPropValue
@param {Object} element
@param {String} propName
@returns Element's property value