<!DOCTYPE html> <html> <head> <style> @namespace url(test); div { background-color: red; } div[mixedCase=true] { background-color: green !important; } </style> <script> function f() { var elem = document.createElementNS("test","div"); elem.setAttribute("mixedCase",true); elem.appendChild(document.createTextNode("text")); document.getElementsByTagName("body")[0].appendChild(elem); } </script> </head> <body onload="f();"> </body> </html>