Ballot on URIs and Namespaces
Ballot on relative URI References in namespace declarations (3-17 July 2000).
Why URI?
The only requirement for a namespace definition is to be unique within all the applications, it possibly can be used in. There is no need to specify a URI scheme, or some www... subdomain.
Namespace URIs
RFC 3986 - Uniform Resource Identifier (URI): Generic SyntaxEach URI begins with a scheme name that refers to a specification for assigning identifiers within that scheme.
Namespace Versioning
 Namespaces and versioning
Using XML namespaces to mark the version of XML formats
 XMLVS
Using Namespace Documents for XML Versioning

Major changes in the schema of the namespace would result in a new version of this namespace. So the namespace URI needs to specify a version.

The version of the schema could also be handled via explicit meta information within the document (eg. version attribute for the root element).

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