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I tried to valid a quite simple document such as <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>Demo</title> </head> <body> <h1>Demo</h1> </body> </html> I got 1 Error and tell me Missing xmlns attribute for element html. The value should be: http://www.w3.org/1999/xhtml and a prompt like this: Many Document Types based on XML need a mandatory xmlns="" on the root element. For example, the root element for XHTML will look like: <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> so I use <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> insted <html> then I got Attribute "lang" exists, but can not be used for this element. ¡ns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> so then what I should write? with lang or without lang?
*** This bug has been marked as a duplicate of bug 6995 ***