<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<style type="text/css">
#box{
border: 1px solid black;
width:500px;
background-color: rgb(100,100,100);
}
.left{
height: 200px;
width: 200px;
border: 1px solid blue;
float: left;
}
.right{
height: 200px;
width: 200px;
border: 1px solid red;
float: right;
}
.clear{
clear: both;
}
#box2{
height: 200px;
border: 1px solid yellow;
background-color: rgba(100,100,100,0.5);
}
</style>
</head>
<body>
<div id="box">
<div class="left"></div>
<div class="right"></div>
<div class="clear"></div>
</div>
<div id="box2">
</div>
</body>
</html>
原文：http://www.cnblogs.com/miceqq/p/4941721.html