FYI: XPath Errata

http://www.w3.org/1999/11/REC-xpath-19991116-errata/
Section 5
... The namespace URI component of an expanded-name is
implementation-dependent if the expanded-name is expanded from a QName
whose prefix is declared by a namespace declaration with a namespace name
that is a relative URI (with or without a fragment identifier). An XPath
expression that depends on the value of the namespace URI component of
such expanded-names is not interoperable.