function measure(el, styleProp) {
    if (el.currentStyle)
         var st = el.currentStyle[styleProp];
    else if (window.getComputedStyle)

    var st = document.defaultView.getComputedStyle(el, null).getPropertyValue(styleProp);
    return st;
}

http://www.quirksmode.org/dom/getstyles.html

Gets the style property as rendered, not as defined (which can be different).

Get the actual style property value of an element

JavaScript