<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<style type="text/css">
.box{
border: 1px solid red;
}
.aa{
width: 200px;
height: 200px;
background: #CCCCCC;
float: left;
}
.clearfix:after{
content: "";
display: block;
clear: both;
}
</style>
</head>
<body>
<div class="box clearfix">
<div class="aa">
</div>
</div>
</body>
</html>
原文：http://www.cnblogs.com/dingdingdown/p/6850802.html