The W3C Technical Architecture Working Group (TAG) has published The Disposition of Names in an XML Namespace. This "document addresses the question of whether or not adding new names to a (published) namespace is a sound practice." Short answer: it depends. Slightly longer answer:
Specifications that define namespaces SHOULD explicitly state their policy with respect to changes in the names defined in that namespace.
For namespaces that are not immutable, the specification SHOULD describe how names may be given definitions (or have them removed) and by whom.
If a namespace document is provided, as [WebArch Vol 1] recommends, the namespace change policy SHOULD be stated in the namespace document.
As a general rule, resources on the web can and do change. In the absence of an explicit statement, one cannot infer that a namespace is immutable.
