4.3 伪元素
可以把样式应用到文档树中根本不存在的元素上。
::first-line 文本中的第一行
::first-letter 文本中的第一个字母
::after 元素之后添加
::before 元素之前
代码：
<!DOCTYPE html>
<html>
<head><meta charset="utf-8"><title></title><style>p::first-line {background-color: red;}p::first-letter {color: blue;font-weight: 600;}h1::before {content: "我是h1前面的添加的内容";color: chocolate;}p::after {content: "我是P后面的内容";color: brown;}</style>
</head>
<body><h1>|WWF's Mission Statement</h1><p>To stop the degradation of theplanet's natural environmentand to build a future in whichhumans live in harmony with nature,by; conserving the world's biological diversity, ensuring that the use of renewable natural resources issustainable, and promoting the reduction of pollution and wasteful consumption.|</p>
</body>
</html>小结：UI元素状态，:selection冷门生僻，不学习。
