试一试 - Bootstrap ref_js_dropdown

<!DOCTYPE html>
<html>
<head>
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="stylesheet" href="/lib/bootstrap.min.css">
  <script src="/lib/jquery-1.12.2.min.js"></script>
  <script src="/lib/bootstrap.min.js"></script>
  <style>
  /* Style to reverse the caret icon from pointing downwards to upwards */
  .caret.caret-up {
    border-top-width: 0;
    border-bottom: 4px solid #fff;
  }
  </style>
</head>
<body>

<div class="container">
  <h2>Change the caret symbol on dropdown</h2>                                       
  <div class="dropdown">
    <button class="btn btn-primary dropdown-toggle" type="button" data-toggle="dropdown">Dropdown
    <span class="caret"></span></button>
    <ul class="dropdown-menu">
      <li><a href="#">HTML</a></li>
      <li><a href="#">CSS</a></li>
      <li><a href="#">JavaScript</a></li>
    </ul>
  </div>
</div>

</body>
</html>