为 id="firstname" 元素添加指定样式 - 在线编辑器

          <!DOCTYPE html>
<html>
<head>
  <style>
    #firstname {
      background-color: yellow;
    }
  </style>
</head>
<body>

  <h1>Welcome to My Homepage</h1>

  <div class="intro">
    <p id="firstname">My name is Donald.</p>
    <p id="hometown">I live in Duckburg.</p>
  </div>

  <p>My best friend is Mickey.</p>

</body>
</html>
        