I think it depends what you define as breaking. I'm a simple, nerdy, backend/infra kinda person, so CSS issues don't really bother me too much - I like your "what gets used, works" thought, if I can read the text, I'll live with it. But I've met stereotypical designer-type web devs who make the 1px gap in Chrome 123 at least sound like a major burden. I don't know Said, but I assume they sit slightly closer towards the "1px gap _is_ broken" side than you or me, even if they're not a full blown designer-type.
Regarding changing expectations, and the mobile example, I find it really interesting to read your thoughts on that. I feel like your website is an exception to normal expectations, at least for me. If I find myself on your site on mobile, I just e-mail the link to myself and pick it up on desktop. The information is so dense, stimulating and interlinked that consuming it on a tiny mobile feels like listening to Mozart over a tin can and some string. I think the physical mobile form factor is not good for long form or cerebral content, but maybe that's my own DeSkToP iS BeStToP bias.
Have you done any analysis on bounce rates based on first visit device? I would guess a slightly higher return rate for people who visit on desktop first, as - to me at least - the archetypal gwern.net desktop website experience is a big part of the charm.
> But if you are being sensible about it and using plain JS (as we do) or well-chosen stable frameworks, of web dev's woes, 'web browsers keep breaking old working CSS/JS' is thankfully far down the list.
I think some of this also depends on motivations for having a website too. If it's to get notes out there, then as long as it can be read it's fine. But if it's to stay up to date and be associated with the latest and greatest web experiences and technologies (or at least be perceived to) you're destined to a life of new-shiny chasing and tech churn. I do wonder how much of that ability to identify "sensible and plain JS" comes from shiny-chasing 10 - 20 years ago and just using the bits that still work from then. What will be the "sensible and plain JS" in 20 years time?
A lot of this I guess is also specific to being a dev for a personal site, rather than pro web designer: part of being a modern business is keeping a modern front, so whilst "browsers breaking because of old stuff" is far down the list "having to switch to new shiny framework because of business decision to 'refresh' things" is a lot higher.
But to try bring to some sort of conclusion, on a technical/presentation level, HTML/CSS/browser breaking stuff are - or will be - problem for a lot of people getting started with personal websites, because even experienced nerds can struggle to identify which tech will be around in 5 years time. You probably don't see it as that much of a problem, because you've solved it with a combination of the 2 options I gave: your site feels like a stylistic ally to motherfuckingwebsite.com (minimalism), and effort has been put in to maintain it over the years (e.g. UI update efforts via Said).
To tie this back to OP about blogs vs wikis, that's a slightly different problem the author faces IMO. Not quite bit rot, but something higher level, an anxiety around content structure matching the shape of thought of the individual author, and the perception of that (blog vs wiki vs garden vs pkm buzzword) rather than technical (React vs Angular vs js/css buzzword).