css div 网页布局 左右定宽,中间根据网页宽度动态变化

<!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>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>css div 网页布局 左右定宽,中间根据网页宽度动态变化</title>
<style>
body{ margin:0; padding:10px;}
#head{ margin-bottom:10px; height:50px; background-color:#999999}
#main{}
#left{ width:200px; float:left;margin-right:-200px; background-color:#FF9900}
#mid{ width:auto;background:#00FF00;margin:0 220px;border:1px solid #000;}
#right{ width:500px;margin-left:-200px; float:right; background-color:#CCCC00}
#foot{ margin-top:10px; clear:both; height:50px; background-color:#CCCCCC}

</style>
</head>

<body>
<div id="head">我是头部</div>
<div id="main">
 <div id="left">我是左边,宽:200px</div>
 <div id="right">我是右边,宽:200px</div>
 <div id="mid">我是中间,宽自适应</div> 
</div>
<div id="foot">我是底部</div>
</body>
</html>