Earlier quoted context omitted.
Is there an article with the background on this? Why do we have both the W3C and the WHATWG, and why do the W3C just copy and paste work from WHATWG, if that is indeed what happens?
Way back in the day, HTML was implemented as an application of SGML. SGML was a quite complex markup format, that had lots of features that were kind of complex to implement, and so web browsers didn't actually implement all of SGML, just that which was necessary for HTML and the HTML found in the wild. However, the HTML found in the wild was frequently invalid, so browsers had to implement some clever rules to do so…
[citation needed]
I'm unaware of basically any implementation of HTML treating it as an application of SGML; the only notable case I'm aware of is the old HTML validator.
Tim's original implementation of HTML didn't treat it as SGML.