131变化两边，固定中间的布局

这里做出的效果是，固定中间不变，左右两边随着浏览器的变化，而变化

实现代码如下：

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style type="text/css">
#header,#content,#footer{
background-color:#ccc;
border:1px solid #00C;
height:50px;
80%;
margin:10px auto;
}
#content{
height:500px;
}
#left{
height:500px;
50%;
float:left;
margin-left:-50px;
overflow:hidden;
}
#right{
height:500px;
49.5%;
float:right;
margin-right:-50px;
overflow:hidden;
}
#cont{
height:500px;
border:1px red solid;
100px;
float:left;
background-color:#0FF;
}
#varleft{
height:500px;
border:1px solid #000;
margin-left:50px;
background-color:#F60;
}
#varright{
height:500px;
border:1px solid #000;
margin-right:50px;
background-color:#F60;
}
</style>
</head>

<div id="content">
    <div id="left">
    <div id="varleft"></div>
     </div>

     <div id="cont">sdfsfsd</div>

    <div id="right">
    <div id="varright"></div>
    </div>
</div>