:after的作用是在当前的应用元素后面添加内容
如div:after{
content:"This is a demo"
}
还有就是清除浮动(参考代码):
<style>
#box{border:2px solid #ccc;}
.clear:after{content:'';display:block;clear:both;}
.box{width:200px;height:100px;background:red;float:left;}
</style>
</head>
<body>
<div id="box">
<div class="box"></div>
<div class="clear"></div>
</div>
</body>