为什么foot不显示啊？
老师帮忙看一下为什么foot不显示啊？
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>一列布局</title>
<style type="text/css">
body{margin:0;padding:0}
.top{height:100px;background-color: #0000FF;}
.main{width:800px;height: 300px;background-color: #FF0000;margin: 0 auto;}
.foot{width:800px,height: 100px;background-color: #00FFFF;margin: 0 auto;}
</style>
</head>
<body>
<div class="top"></div>
<div class="main"></div>
<div class="foot"></div>
</body>
</html>
