Back in t get late 90ies I wrote my markup in XML and be then used mod_perl to transform it to XHTML using XSLT on the fly. But to be extra 1337, I renamed my index.html file to index.xml,
I contributed to a monstrosity based on Xalan and XSLT at runtime. What a beast. I don't know how we planned to support 5 concurrent users.
Btw the mod_perl XML server was called AxKit. I don’t think anyone even remembers it.
I do remember the name, but I never used it or got into direct contact with it.