I think XHTML is a subset of XML that defines tags in HTML. Strictly, there should be an XML::XHTML:: namespace. However, HTML and XML also happen to be subsets of SGML, but nobody made SGML::HTML:: or SGML::XML:: namespace, so there is at least precedent for making very-popular subsets into a namespace of its own.

I suspect that XHTML has quite a specific purpose and is widely used enough that it should be given its own namespace. The XML:: namespace seems mostly filled with parsers or generators, and not with specific uses of XML.

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I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

Note: All code is untested, unless otherwise stated