1. <body>
    3.     <div id="app">app</div>
    4.     <div class="aaa">aaa</div>
    5.     <button>哈哈</button>
    7.     <ul>
    8.         <li>1</li>
    9.         <li>2</li>
    10.         <li>3</li>
    11.     </ul>
    13.     <script src="https://cdn.bootcdn.net/ajax/libs/jquery/3.6.0/jquery.min.js"></script>
    14.     <script>
    15.         //$('#app')
    16.         //$('.aaa')
    17.         //$('button')
    18.     </script>
    19. </body>