XML "Namespaces" are vocabularies. The XML "Namespace" recommendation is a step on the road toward interoperability for XML messages.
A "namespace" amounts to an abstract "place" where there is a list of element type names (tag names) and/or attribute names.
URIs specify the namespaces in use.
There is no requirement that the specified URI is valid, much less that the indicated resource conforms to any sort of specification.
XML "Namespaces" provide a way for names to be guaranteed to be unique, and that's all.