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<!DOCTYPE html> <html> <head> <style> .democlass { color: red; } </style> </head> <body> <h1>Hello World</h1> <p>Click the button to set the class attribute node of the header above.</p> <button onclick="myFunction()">Try it</button> <script> function myFunction() { var attr = document.createAttribute("class"); attr.value = "democlass"; var h = document.getElementsByTagName("H1")[0]; h.setAttributeNode(attr); } </script> </body> </html>