如果不使用position:absolute定位，而采用float:left，为什么左栏会被挤到下面？
<!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{height:100px; margin:0 auto; background:#ccc;}
.main{height:800px;margin:0 auto; background:red; position:relative;}
.left{width:200px; height:100%; background:blue; position:absolute; left:0; top:0;}
.right{height:100%; margin-left:210px; background:green;}
.foot{height:70px; margin:0 auto; background:orange;}
</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>
