<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style type="text/css">
li:last-child
        {
color: red;
        }
.box1 p:last-child
        {
color: blue;
        }
.box1 p:first-child
        {
color:yellow;
        }
    </style>
</head>
<body>
    <ul>
        <li>a1</li>
        <li>a2</li>
        <li>a3</li>
        <li>a4</li>
        <li>a5</li>
    </ul>
    <div class="box1">
        <p>1</p>
        <p>2</p>
        <p>3</p>
        <p>4</p>
    </div>
</body>
</html>

核心：first-child：第一个，所在元素的哦

last-child：最后一个，所在元素的哦