W3School TIY Editor

<!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.jsdelivr.net/npm/bootstrap@5.1.2/dist/css/bootstrap.min.css" rel="stylesheet">
  <script src="https://cdn.jsdelivr.net/npm/bootstrap@5.1.2/dist/js/bootstrap.bundle.min.js"></script>
  <style>
  span {
    display: inline-block;
    width: 70px;
    height: 70px;
    margin: 6px;
    background-color: #f9f9f9;
  }
  </style>
</head>
<body>

<div class="container mt-3">
  <h2>边框</h2>
  <p>使用边框类为元素添加或删除边框：</p>

</body>
</html>