实例演示

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel="stylesheet" href="https://cdn.staticfile.org/twitter-bootstrap/3.4.1/css/bootstrap.min.css">
  </head>

<body>
    <div class="container">
      <h2>Volume-down Glyph</h2>
      <p>Volume-down icon: <span class="glyphicon glyphicon-volume-down"></span></p>    
      <p>Volume-down icon as a link:
        <a href="#">
          <span class="glyphicon glyphicon-volume-down"></span>
        </a>
      </p>
      <p>Volume-down icon on a button:
        <button type="button" class="btn btn-default btn-sm">
          <span class="glyphicon glyphicon-volume-down"></span> Volume-down
        </button>
      </p>
      <p>Volume-down icon on a styled link button:
        <a href="#" class="btn btn-info btn-lg">
          <span class="glyphicon glyphicon-volume-down"></span> Volume-down
        </a>
      </p> 
    </div>

</html>