Javascript 获取css属性问题

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title> new document </title>
    <meta name="generator" content="editplus" />
    <meta name="author" content="Gao YiXiang" />
    <meta name="email" content="[email protected]" />
    <meta name="keywords" content="javascript dhtml dom" />
    <meta name="description" content="I love web development." />
    <style> 
#L 
{
    position:absolute;
    color:#1c4a74;
    background-color:#f3faff;
    border:1px #bfd1eb solid;
    top:101px;
    left:101px;
}
</style>
</head>
<body><div id="L">123</div><script> 
var obj = document.getElementById("L"); 
var _left = obj.currentStyle.left; 
var _top = obj.currentStyle.top; alert(_left + " - " + _top) 
</script> </body>
</html>