I'm not tracking what's currently going on with the underwater portion of frontend iceberg. Are things still like it was in late '00s and early '10s, where browsers still had plenty of their unique implementation quirks and non-standard features, and plenty of sites were relying on those?

Back in the day, it was not entirely unheard of having two significantly different frontend implementations - one for IE, another for Netscape, with quite unhealthy amounts of parser hacks to hide code from the browsers.

Possibly naively, but I think it's not that bad nowadays? (At least it wasn't so in late '10s.) Some things are Chrome-only, or Apple-only, but I rarely see "not supported in your browser" - the majority of features is generally standards compliant, and all those newcomer engine problems (like in the article) are mostly because there's a lot to implement.

Twitch randomly blocks Chromium.

(They don't even block the streams, only logins. I can watch, but not spend money. Perfectly reasonable.)

Okay but if everyone building the websites are using the browser with 0.3% market share it won't matter.