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 the
planet's natural environment
and to build a future in which
humans live in harmony with nature,
by; conserving the world's biological diversity, ensuring that the use of renewable natural resources is
sustainable, 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。
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