A proposal

Requirements:

[a] All string-equal namespace URI must be namespace-equal.
[b] All absolutized-equal namespace URI must be namespace-equal.
[c] If namespace compliance is redefined, then current documents
    must remain in compliance and interpretable as they are now.

Solutions:

[A] Literal interpretation.  Supports requirements [a] and [c].
[B] Absolutized interpretation.  Supports requirements [b] and [c]
    if implicit contract of static hierarchical relationship between
    document and resource referred to by NSURI is upheld/upholdable
    by owner/producer; otherwise, supports requirement [b] only.
[C] Forbid relative URIs.  Supports requirements [a] and [b].
    
Proposal:

[D] Solution [B], but with absolutization redefined for namespaces,
    such that it is with respect to a namespace-specific base URI
    specifiable via a document-level attribute with a default
    value, e.g. 
    
       xmlbase:xmlns #IMPLIED "urn:xmlns:"
       
    Supports all requirements.


Respectfully,
Steve Rowe
MNIS-TextWise Labs