I think this was the original vision of XML: eXtensible markup language. With globally unique names XML was meant to produce a whole world of languages that could've been developed independently and mixed together in the same manner as Unix command-line tools. E.g. XSLT is a language for a XSLT processor; XSL-FO is a language for XSL-FO processor and so on. In this schema XHTML would've been a language for the browser that would co-exist with the rest of the ecosystem.