What browsers made out of it isn't the matter here, but even if it were, the "practical, real-world HTML out there" argument is mostly used to pull up the ladder by an ad company/browser cartel made worse day-in day-out through an atrocious and absurdly voluminous HTML spec (and by CSS, of course).
Even though Ian Hickson, of WHATWG, wanted to capture HTML as it was understood by browsers, he couldn't help but added additional elements of his own - such as for marking up ads as "aside" lol plus the alien sectioning elements concept that gave rise to the flawed "outline algorithm" and misuse of heading elements (and earlier failure to understand SGML's RANK feature), a problem that was only fixed last year [2] by an incompatible change to HTML invalidating documents using hgroup as originally advised.
In practice, very few changes to the HTML syntax brought HTML outside SGML - for the most part, ad-hoc and basically unnecessary commenting rules for the script and style elements to keep legacy browsers from rendering JavaScript and CSS, resp., when those where introduced.