高度自适应问题
<!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{width: 100%;height: 20%; background: mediumvioletred;}
.main{width: 100%;height: 70%; background: red;}
.left{width: 25%;height: 100%; background: burlywood;float: left;}
.right{width: 74%;height: 100%; background: blue; float: right;}
.foot{width: 100%;height: 10%;background: lawngreen;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>
为什么高度这样写自适应不可以，达不到效果
