<!DOCTYPE html>
<html lang="en">
<head>
<title>Bootstrap 示例</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://www.w3schools.cn/lib/bootstrap/3.4.1/css/bootstrap.min.css">
<script src="https://ajax.aspnetcdn.com/ajax/jQuery/jquery-3.5.1.min.js"></script>
<script src="https://www.w3schools.cn/lib/bootstrap/3.4.1/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container-fluid">
<h1>Hello World!</h1>
<div class="row">
<div class="col-xs-6 col-sm-3" style="background-color:lavender;">
第1栏<br>
调整浏览器窗口大小以查看效果。 也尝试删除 div clearfix 行，看看会发生什么。
</div>
<div class="col-xs-6 col-sm-3" style="background-color:lavenderblush;">第2栏</div>
<div class="clearfix visible-xs"></div>
<div class="col-xs-6 col-sm-3" style="background-color:lightcyan;">第3栏</div>
<div class="col-xs-6 col-sm-3" style="background-color:lightgray;">第4栏</div>
</div>
</div>
</body>
</html>