HTML5 has an XML “serialization” (as the spec calls it), which handles the role that XHTML did previously. It’s mostly useful for embedding into other XML docs, but I’ve never seen it in the wild for creating web pages. It’s worth noting that HTML5 is its own thing now, and is not even SGML-compliant like previous versions were.
You could always teach yourself some XML and dabble with the XML serialization of HTML5, but other than satisfying curiosity, there’s really not much general use you’re going to get out of it. I think you’ll find with both legacy XHTML and the XML serialization that you’ll find fewer things it’s compatible with, not more.