function getComputedStyle(element, prop) {
            if (element.currentStyle) {
                return element.currentStyle[prop];
            } else if (window.getComputedStyle) {
                return window.getComputedStyle(element, null).getPropertyValue(prop);
            } else {
                return element.style[prop];
            }
        }
@param {HTMLElement} element
@param {String}      prop
@returns {String|Number}