This goes to show you’ve never been anywhere near the actual development cycle of a real-world front-end web application. “So, there is a baseline "target subset of HTML/CSS" that gives you 100% coverage.” Oh really? Which subset? Which “HTML/CSS?” And 100%? Absolutely laughable.

Do you know any browsers which don’t support https://motherfuckingwebsite.com/ (if you remove the google traffic tracking js that’s iirc tacked on at the end of the page (or maybe I’m thinking of better mfing website (which adds a tiny bit of css)? Idr.)) ?

I get that asking a commercial website to be as basic/supported as that website is a big ask. I don’t think the other commenter was saying that such websites should reach 100%, only that they should start from there and sacrifice only as much as is necessary.