为什么main的颜色不显示，不应该left和right之间是黑色的吗，为什么是白色的呢？
<!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>二列布局</title>
<style type="text/css">
body{padding:0;margin:0;}
div{ text-align:center; line-height:50px;}
.main{background-color:#000;width:100%;margin:0 auto;}
.left{background-color:blue;width:20%;height:600px;float:left;}
.right{background-color:#CF6;width:79%;height:600px;float:right;}
</style>
</head>
<body>
<div class="main">
<div class="left">left</div>
<div class="right">right</div>
</div>
</body>
</html>
