效果：
代码：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>伪元素选择器</title>
<style type="text/css">
.head{
margin: 0 0 0 100px;
height: 80px;
}
.container{
margin: 0 0 0 100px;
}
p{
400px;
height: 60px;
border: 1px solid red;
position: relative;
top: 0;
left: 0;
}
div{
400px;
height: 200px;
margin: 20px 0 0 0;
}
p::before{
position: relative;
display: block;
top: 20px;
left: 20px;
0;
height: 0;
float: left;
content: "";
border: 10px solid red;
border-color: red transparent transparent transparent;
}
p::after{
position: relative;
display: block;
top: 20px;
right: 20px;
0;
float: right;
height: 0;
content: "";
border: 10px solid red;
border-color: red transparent transparent transparent;
}
.div1:first-letter{
font-weight: bold;
color: red;
font-size: 30px;
}
.div2:first-line{
font-weight: bold;
color: red;
font-size: 30px;
}
.div3::selection{
color: white;
background-color: black;
}
.div3::-moz-selection{
color: white;
background-color: black;
}
.div3::-webkit-selection{
color: white;
background-color: black;
}
</style>
</head>
<body>
<div class="head">
<h1>伪元素选择器</h1>
<h2><a href="inde.html">首页</a></h2>
</div>
<div class="container">
<p>::before,::after</p>
<div class = 'div1'>
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</div>
<div class = 'div2'>
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</div>
<div class = 'div3'>
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</div>
</div>
</body>
</html>
总结：
1、first-letter： 获取第一个字母或中文字；
2、first-line：给block，inline-block，table-capttion，table-cell元素提供选取第一行元素的功能；
3、::before,::after：给元素添加额外的块；
4、selection：被选中的文本内容。