I don't disagree with any of that (and excellent work; the Wild West days were hard to get past)...
... But we both arrive at the same place, where if a site works on incumbent browsers (by spec or by shared quirk because in an ambiguity of the spec everyone lucked into the same implementated behavior) and not an outlier browser, and the site is popular, users perceive the outlier to be broken. Because users don't understand this problem space by parsing specs; they understand it as "well it works on my sister's computer when she double-clicks the rainbow circle; I guess the compass just doesn't work with the whole web. Maybe I should just get myself a rainbow circle." Hence, the existence of a Quirks.cpp file.
> But we both arrive at the same place, where if a site works on incumbent browsers (by spec or by shared quirk because in an ambiguity of the spec everyone lucked into the same implementated behavior) and not an outlier browser, and the site is popular, users perceive the outlier to be broken
The difference between the past and now, is that it is very well understood by all the major vendors (gecko, blink, webkit) that if the spec has a section where different behaviour is permitted - other than things that are necessarily non-deterministic (networking, timers within some bound, ...), or where platform behavior is different as with IMEs - the specification itself is broken. Similarly if the spec disagrees what browser engines are actually doing, the spec is wrong. Once an issue is identified, the spec is then fixed, regardless of effort, to ensure that the gaps are filled out and the errors corrected.
The point is that if a new browser comes along, and correctly implements the spec, that browser should work with the same content as any other engine, and if it can't the spec is broken. This is the model the engine developers want. Yes it may cause them to face new competition, but having a complete spec is a massive enabler. It lets you make massive internal changes to your engine without having to worry about "are there sites that depend on X"[1].
Now, even when there are gaps or errors in the spec such that observable implementation details leak out, if a developers site only works in one browser (sans browser bugs, which please file bugs, the engineers at all these companies do care, and do value them) that site is depending on unspecified behavior, so the site is wrong.
From an end-users point of view, yes it appears that other browsers are broken, but that isn't the problem.
The problem is that the web developer turns around and says "it's not my site that is broken, it's the other browsers". This is the development model that means fundamentally Chrome is the new IE: it is the only browser that is resulting in sites saying "you need Chrome to continue" or developers saying "it works in Chrome but not X, X must be wrong" and not considering any alternative.
[1] Obviously there are quirks as listed in this file, but you can see that the size of this file and a couple of similar ones are very small, and the quirks are exceptionally specific, essentially they are as close as possible to "Apply this quirk, to this site, only if the site is still using this specific design/layout". In the past engines essentially had to go "we've got one site depending on this behavior, which has no real specification so we need to guess whether this is the actual behavior we should have, or whether it's uncommon, or even just a one off".
... But we both arrive at the same place, where if a site works on incumbent browsers (by spec or by shared quirk because in an ambiguity of the spec everyone lucked into the same implementated behavior) and not an outlier browser, and the site is popular, users perceive the outlier to be broken. Because users don't understand this problem space by parsing specs; they understand it as "well it works on my sister's computer when she double-clicks the rainbow circle; I guess the compass just doesn't work with the whole web. Maybe I should just get myself a rainbow circle." Hence, the existence of a Quirks.cpp file.