效果图：
:一个冒号是css的过时用法
::两个冒号是css3的用法
<!DOCTYPE html>
<html lang="en">
<style>
h2::before {/*一个:是css过时语法的，2个:是CSS3语法/
/* content: 'hi'; */
content: '';
color: palegreen;
width: 200px;
margin: 8px;
display:inline-block;
border: 1px solid rebeccapurple
}
h2::after {
content: '';
color: palegreen;
width: 200px;
margin: 8px;
display:inline-block;
border: 1px solid rebeccapurple
}
</style>
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<h2>精品选购</h2>
</body>
</html>
