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<!DOCTYPE html> <html> <head> <style> .democlass { color: red; } </style> </head> <body> <h1>Hello World</h1> <p>单击按钮创建一个值为“democlass”的“class”属性，并将其插入到上面的H1 元素中。</p> <button onclick="myFunction()">试一试</button> <script> function myFunction() { var h1 = document.getElementsByTagName("H1")[0]; var att = document.createAttribute("class"); att.value = "democlass"; h1.setAttributeNode(att); } </script> </body> </html>