<!DOCTYPE html> <html> <style> .democlass { color: red; } </style> <body> <h1 id="myH1">Element 对象</h1> <h2>setAttribute() 方法</h2> <p>点击“添加类”为 h1 元素添加 class 属性:</p> <button onclick="myFunction()">添加类</button> <script> function myFunction() { document.getElementById("myH1").setAttribute("class", "democlass"); } </script> </body> </html>