You're right about the fragmentation issue - that is something that conceivably stop someone outputting valid X/HTML. But that would be the kind of implementation issue that a XHTML:: module should support - chaining of object serialization, for example, could solve the problem in a nice way.

My hope for anytbing XHTML:: would not be that it would output mostly XHTML, but that it would guarantee to output valid XML at least. That would be the selling point for me. Unfortunately, XML is a little unwieldy in Perl in some respects (thinking mostly about accessibility of modules on common systems), but what the hey.

I don't think many people get the benefits of XHTML yet. For example, I had validated my site as XHTML for quite a while before delivering the content with the correct MIME type. Mozilla had a big surprise in wait for me when I did that - it doesn't render non-XML pages. That is why an XHTML:: module must guarantee XML output - you can't actually do 'correct' XHTML without it (or, you can try, but Mozilla won't display anything other than an error message).