<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><title>Title</title><style>/* ul的第一个子元素 */body ul li:first-child{background: #12ec4e;}/* ul的最后一个子元素 */ul li:last-child{background: red;}</style>
</head>
<body><p>p1</p><p>p2</p><p>p3</p><ul><li>li1</li><li>li2</li><li>li3</li></ul></body>
</html>
此外还有p:nth-child(2)，p:nth-of-type(1)
https://www.bilibili.com/video/BV1YJ411a7dy?p=7
