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Is "xmlns:xml" legal or not?

Recently, I've run into a compatibility problem with some XML tools over whether it is valid to have an "xmlns:xml" declaration in a document.  On close reading, the Namespaces spec says that namespace prefixes beginning with the (regular expression) pattern "[Xx][Mm][Ll]" are reserved, but it doesn't say that they are not allowed.  Nonetheless, this has led to two different interpretations by tool vendors:

1. Flag all namespace prefixes starting the "[Xx][Mm][Ll]" as an error;
2. Allow "xmlns:xml" to be declared, but only allow it to be associated with the namespace URI "http://www.w3.org/XML/1998/namespace" (and do who knows what else for any other prefix starting with "[Xx][Mm][Ll]".

I think we need to get a consensus on this one, and then (as necessary) push to have it added as an erratum to the Namespaces spec.  This is too big a crack to leave unfilled, because if one tool adds "xmlns:xml" to a document, and another flags it as an error, less erudite users are just going to think that XML and/or XML tools are unreliable, and I don't want to see that happening (life's too short, etc.).

As for my preference, having worked with tools that implement each interpretation, I think that allowing "xmlns:xml" to be defined (but only associated with a fixed [XML-version-dependent?] namespace URI) if actually easier for new users to understand than the alternative, that you may have to import the XML namespace to use "xml:lang", but you must not declare the prefix as you do for every other namespace.

You thoughts and comments (to this list) would be very welcome.

Anthony B. Coates
(1) Content Distribution Architect - Project Gazelle
(2) Leader of XML Architecture & Design - Chief Technology Office
Reuters Plc, London.

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