not inline css style
http://stackoverflow.com/questions/12010506/how-to-get-the-css-property-from-external-css-file-if-the-inline-style-is-applie
by lalatino
HTML
<div id="my" style='left:100px'>some content</div>
CSS
#my{left:250px;}
JavaScript
var el = document.getElementById('my').cloneNode(false);
el.removeAttribute('style');
var left = window.getComputedStyle(el, null).getPropertyValue('left');
alert(left);