The createAttributeNS is supposed to throw the DOMException.NAMESPACE_ERR for the following cases

namespace uri qualified name
------------- --------------
null prefix of xml
empty string prefix of xml
other than http://www.w3.org/XML/1998/namespace" prefix of xml
null prefix of xmlns
empty string prefix of xmlns
other than http://www.w3.org/2000/xmlns/" prefix of xmlns

It does not throw exception for these cases.