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<!DOCTYPE html> <html> <head> <style> .democlass { color: red; } </style> </head> <body> <h1>Hello World</h1> <p>单击该按钮可将值为“democlass”的类属性添加到 h1 元素。</p> <button onclick="myFunction()">试一试</button> <script> function myFunction() { document.getElementsByTagName("H1")[0].setAttribute("class", "democlass"); } </script> </body> </html>