Comment by dragonwriter

3 hours ago

No, HTML was a specific application profile of SGML (modern HTML, I believe, no longer technically is), XML is a newer (than HTML) application profile of SGML inspired by HTML but aiming for greater generality.

XHTML was an attempt to encode HTML semantics (approximately, each version of XHTML also altered some semantics from HTML and previous XHTML versions) in XML, and the XML serialization of modern, WHATWG HTML exactly encodes HTML semantics in XML.