js中如何去获取外部css样式

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style type="text/css">
body{
margin: 0;
padding: 0;
background-color: #e6e6e6;
}

.container{
width: 768px;
margin: auto;
padding: auto;
background-color: #FFFFFF;
}

.head{
height: 80px;
}

.nav{
height: 45px;
}

.main{
background-color: #00FF00;
}

.foot{
width: 100%;
height: 45px;
background-color: #FF0000;
z-index: 1;
}


.main .left{
float: left;
width: 200px;
height: 600px;
background-color: #E5EECC;
}

.main .right{
float: left;
width: 568px;
height: 600px;
background-color: #EFEFEF;
}

.clearboth{
clear: both;
height: 0px;
}

</style>
<script type="text/javascript" src="js/jquery/jquery-1.9.1.min.js" ></script>
<script type="text/javascript">
window.onload=function(){
var main=document.getElementById("main");
main.onclick=function(){
var mainStyle = main.currentStyle? main.currentStyle : window.getComputedStyle(main, null);//加入此方法便可解决
var w1=mainStyle.width;
alert(w1);
}
}
</script>
</head>
<body>
<div class="container">
<div class="head"></div>
<div class="nav"></div>
<div id="main" class="main">
<div class="left"></div>
<div class="right"></div>
<div class="clearboth"></div>
</div>
<div class="foot">foot</div>
</div>
</body>


</html>