link标签引入，写在link标签中，有条件的引入外部样式表
index.html:
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><link rel="stylesheet" href="css-1.css"><link rel="stylesheet" href="css-2.css"media="(min-width:400px)"><link rel="stylesheet" href="css-3.css"media="(min-width:300px) and (max-width:399px)"></head><body><div class="div0"><div></div><div></div><div></div></div></body></html>css-1.css:.div0{width:100%;height:500px;border:1px solid #ccc;}/* 子div浮动一行显示 */.div0 div{float:left;height:200px;}.div0 div:nth-child(1){background-color: red;}.div0 div:nth-child(2){background-color: green;}.div0 div:nth-child(3){background-color: blue;}css-2.css:/* 一行显示3个div */.div0 div{width: 33.3%;}css-3.css:.div0 div{width: 50%;}
