<html>
<body>
<input type=button value='aff position'
onClick="alert(document.getElementById('foo').style.left)"><br>
<input type=button value='move 20px'
onClick="
var St = document.getElementById('foo').style;
var ll = parseInt(St.left);
St.left = ll+20;
">
<div id='foo' style='position:absolute; top:100px; left:10px;'>
foobar
</div>
</body>
</html>