http://www.w3.org/TR/REC-xml-names/

[1] NSAttName ::= PrefixedAttName | DefaultAttName
[2] PrefixedAttName ::= 'xmlns:' NCName
[3] DefaultAttName ::= 'xmlns'
[4] NCName ::= Name - (Char* ':' Char*) /* An XML Name, minus the ":" */

何で規格読まないの?バカなの?