> Regarding DOCTYPE and DTDs, browsers at best made use of those to switch into or out of "quirks mode", on seeing special hardcoded public identifiers but ignored any declarations.
Not when processing XML mime types. In modern browsers that mostly means SVG files, but i think XHTML is still possible.
(Modern) HTML is neither SGML nor XML, so it doesn't follow the rules of either.
"Modern" WHATWG HTML is still following SGML rules to the letter in its dealings with tag inference and attribute shortforms ([1]). Which isn't surprising when it's supposed to hold up backward compat. To say that "HTML is not SGML" is a mere political statement so as not be held accountable to SGML specs. But (the loose group of Chrome devs and other individuals financed by Google to write unversioned HTML spec prose that changes all the time, and that you're calling "modern HTML" even though it doesn't refer to a single markup language) WHATWG had actually better used SGML DTDs or other formal methods, since their loose grammar presentation and its inconsistent, redundant procedural specification in the same doc is precisely were they dropped the ball with respect to the explicitly enumerated elements on which to infer start- and end-element tags. This was already the case with what became W3C HTML 5.1 shortly after Ian Hickson's initial HTML 5 spec (which captured SGML very precisely) ([1]). But despite WHATWG's ignorance, even as recent as two or three years ago, backward compatibility was violated [2]. Interestingly, this controversity (hgroup content model) showed up in a discussion about HTML syntax checkers/language servers just the other day ([3]).
Where HTML does violate SGML was when CSS and JS were introduced already, to prevent legacy browsers displaying inline CSS or JS as content. The original sin being to be place these into content rather than attributes or strictly into external resources in the first place.
Regarding SVG and XHTML, note browsers basically ignore most DTD declarations in those.
> "Modern" WHATWG HTML is still following SGML rules to the letter...To say that "HTML is not SGML" is a mere political statement so as not be held accountable to SGML specs.
That is self-contradictory and makes no sense. If its following sgml to the letter than there is nobody to be held accountable for violating the sgml spec and hence nobody to hide behind "political statements".
You can't have this both ways.
> Regarding SVG and XHTML, note browsers basically ignore most DTD declarations in those.
They listen to dtd's for entity references and default attribute values. I'd hardly call that ignoring.
Not when processing XML mime types. In modern browsers that mostly means SVG files, but i think XHTML is still possible.
(Modern) HTML is neither SGML nor XML, so it doesn't follow the rules of either.