I stand corrected: HTML was defined as an SGML application from the very first published version in 1993 (https://www.w3.org/MarkUp/draft-ietf-iiir-html-01.txt), but I know the original draft in 1990-91 was heavily SGML inspired even if it didn't really conform to the spec (nor provide a DTD). Thanks for pointing this out, it's funny how memory can play games on us :)
While HTML is clearly the most used document markup language there has ever been, almost nobody is using an SGML-compliant parser to parse and process it, and most are not even bothering with the DTD itself; not to mention that HTML5 does not provide a DTD and really can't even be expressed with an SGML DTD.
So while HTML used to be one of SGML "applications" (document types, along with a formal definition), on the web it was never treated as such, but as a very specific language that is inspired by SGML and only inspired by the spec too (since day 1, all browsers accepted "invalid" HTML and they still do).
Ascribing the success to SGML is completely backwards, IMHO: HTML was successful despite it being based on SGML, and for all intents and purposes, majority never really cared about the relationship.
While HTML is clearly the most used document markup language there has ever been, almost nobody is using an SGML-compliant parser to parse and process it, and most are not even bothering with the DTD itself; not to mention that HTML5 does not provide a DTD and really can't even be expressed with an SGML DTD.
So while HTML used to be one of SGML "applications" (document types, along with a formal definition), on the web it was never treated as such, but as a very specific language that is inspired by SGML and only inspired by the spec too (since day 1, all browsers accepted "invalid" HTML and they still do).
Ascribing the success to SGML is completely backwards, IMHO: HTML was successful despite it being based on SGML, and for all intents and purposes, majority never really cared about the relationship.