2.1.1 [XPATH] Section 5, Data Model
The specification states:
The namespace URI specified in the XML document can be a URI reference as defined in [RFC2396]; this means it can have a fragment identifier and can be relative. A relative URI should be resolved into an absolute URI during namespace processing: the namespace URIs of expanded-names of nodes in the data model should be absolute. Two expanded-names are equal if they have the same local part, and either both have a null namespace URI or both have non-null namespace URIs that are equal.
MSXML3 and MSXML6
A relative namespace URI is not resolved into an absolute URI during namespace processing.