> XML [...] was not [...] directly related to SGML in any way
That's incorrect. XML is by definition a proper subset of WebSGML, the SGML revision specified in ISO 8879:1986 Annex K. These two specifications were published around the same time and authored by the same people.
In a nutshell, XML added DTD-less SGML (eg. such that every document can be parsed without markup declarations, unlike eg. HTML which has `img` and other empty elements the parser needs to know about) and XML-style empty elements. The features removed from SGML to become XML were tag inference/omission (as used in HTML), short references (for things such as Wiki syntax, CSV, and even JSON parsing), uses of marked sections other than `CDATA`, more complex use cases for notations, and link process declarations ("stylesheets") plus a couple others.
That's incorrect. XML is by definition a proper subset of WebSGML, the SGML revision specified in ISO 8879:1986 Annex K. These two specifications were published around the same time and authored by the same people.
In a nutshell, XML added DTD-less SGML (eg. such that every document can be parsed without markup declarations, unlike eg. HTML which has `img` and other empty elements the parser needs to know about) and XML-style empty elements. The features removed from SGML to become XML were tag inference/omission (as used in HTML), short references (for things such as Wiki syntax, CSV, and even JSON parsing), uses of marked sections other than `CDATA`, more complex use cases for notations, and link process declarations ("stylesheets") plus a couple others.