代码示例：（标识：bs_flex-responsive）
<!DOCTYPE html>
<html lang="en">
<head>
<title>Bootstrap 实例</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link href="https://cdn.bootcdn.net/ajax/libs/twitter-bootstrap/5.1.2/css/bootstrap.min.css" rel="stylesheet">
<script src="https://cdn.bootcdn.net/ajax/libs/twitter-bootstrap/5.1.2/js/bootstrap.bundle.js"></script>
</head>
<body>
<div class="p-4">
<h2>弹性框</h2>
<p>如需创建 flexbox 容器，请添加 d-flex 类。请使用任何 d-*-flex 类来控制元素何时应该是特定屏幕宽度上的 flex 元素。</p>
<p>请调整浏览器窗口大小来查看效果。</p>
<div class="d-flex bg-success">d-flex</div>
<span class="d-sm-flex bg-success">d-sm-flex</span>
<span class="d-md-flex bg-success">d-md-flex</span>
<span class="d-lg-flex bg-success">d-lg-flex</span>
<span class="d-xl-flex bg-success">d-xl-flex</span>
<span class="d-xxl-flex bg-success">d-xxl-flex</span>
</div>
</body>
</html>
