:befor、:after是CSS的僞元素，什麼是僞元素呢？僞元素用於向某些選擇器設置特殊效果。
我們用CSS手冊可以查詢到其基本的用法：
- E:before/E::before 設置在對象前（依據對象樹的邏輯結構）發生的內容。用來和content屬性一起使用
- E:after/E::after 設置在對象後（依據對象樹的邏輯結構）發生的內容。用來和content屬性一起使用
- Ie6-7 不支持
<!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=gbk" />
<title>css_brfor_after</title>
<style type="text/css">
body {
width: 300px;
height: auto;
display: block;
margin: 0px auto;
padding: 0px;
text-align: left;
text-decoration: none;
background: #efeeea;
background: linear-gradient(#f9f9f9, #cecbc4);
background: -moz-linear-gradient(#f9f9f9, #cecbc4);
background: -webkit-linear-gradient(#f9f9f9, #cecbc4);
background: -o-linear-gradient(#f9f9f9, #cecbc4);
}
#box {
width: 300px;
height: 300px;
}
#box .relative {
width: 288px;
height: 288px;
margin-left: 5px;
margin-top: 5px;
background-color: transparent;
border-radius: 5px;
-moz-border-radius: 5px;
border: 2px solid red;
position: absolute;
}
#box .relative:before {
width: 288px;
height: 248px;
background-color: transparent;
border-left: 2px solid #fff;
border-right: 2px solid #fff;
position: absolute;
content: '';
left: -2px;
top: 20px;
}
#box .relative:after {
width: 248px;
height: 288px;
background-color: transparent;
border-top: 2px solid #fff;
border-bottom: 2px solid #fff;
position: absolute;
content: '';
top: -2px;
left: 20px;
}
</style>
</head>
<body>
<div id="box">
<div class="relative"></div>
<img src="ba.png" width="300" height="300" alt="心" title="心" />
</div>
</body>
</html>
