Nothing -- but the point is that there's nothing preventing me
from doing anything else either! Hence, no standard: I can't
look at a document and know what its namespaces "mean" in any
useful sense. (With DTDs, one expected natural language text
in the form of per-element/attribute/... comments.)
Perhaps what we need is XML Processing Instructions to associate
with the namespace declarations, which identify how the bindings
between namespace URIs and semantics are provided ... ;-)
Yes, I know that defining "meaning" was not a goal of the XML
namespace team.
- Dave