为什么footer不见了?
<!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>混合布局编程挑战</title>
<style type="text/css">
body{ margin:0; padding:0; font-size:30px; color:#fff}
.top{margin:0 auto; width:100%; background:#ccc;height:100px;}
.main{margin:0 auto; width:100%; background:#ffc;height:600px;position:absolute;}
.left{float:left; width:200px; background:#fcc;height:600px; position:absolute;top:0;}
.right{background:#cff;height:100%; margin-left:210px;}
.foot{margin:0 auto; width:100%; background:#f99;height:100px;clear:both;}
</style>
</head>
<body>
<div class="top">top</div>
<div class="main">
<div class="right">right</div>
<div class="left">left</div>
</div>
<div class="foot">foot</div>
</body>
</html>