It was immediately closed as off topic, and there were a bunch of extremely vitriolic comments offended that I'd ask such a question on SO. It was briefly reopened weeks (?) later and then I guess closed again and now is deleted, so you can't even view the question any more.
I'd long heard of abusive moderation but... experiencing it first hand is something else. Anecdote of one, but I know I'm never going to ask there again.
In case anyone's wondering, I ended up asking on the WhatWG or W3C or something github project (via an issue?). The TLDR was rather eye opening, that basically the spec only codifies points of contention for browsers and old behaviors are generally undocumented. With some pointers I figured out the default size behavior through code diving, and it was complex (as in, hard to use) and very unintuitive.