XML is a profile of an ISO standard SGML, and most of XML comes from SGML unchanged.
HTML is a subset of SGML and XML is a highly functional subset of SGML.
XHTML extends and subsets HTML.
Question: Does "one being a subset of another" mean that code in the first is also syntactically correct and semantically the same as in the second?
As in the sense of elementary set theory,
- are HTML, XML and XHTML all different subsets of SGML?
- do XML and HTML almost not intersect each other?
- is XHTML a superset of both XML and HTML?
XML is not a single Markup Language but a metalanguage to let users design their own markup language.
I was wondering how to understand XML and HTML are both subsets of SGML,
but HTML is a markup language while XML is not a markup language but a metalanguage for designing markup languages?
Question: Are SGML and XHTML both also metalanguage for designing markup language?
As in both links mention that HTML is an applicaiton of SGML as well as a subset of SGML, and XHTML is an application of XML.