> That's not much better than "everyone runs Windows"

It's absolutely better. It's open standards based instead of proprietary, all current engines are opensource, there are multiple implementations.

> select few browsers

There are three current engines, and I try https://ladybird.dev/ every now and then. It's gotten pretty good and proves that even with a small team you can write a browser that will work with many sites.

> And there will certainly be users who just want to avoid the low-contrast always-animating nature of some sites, for example.

You could make all of that without JS. I'm not sure why people keep conflating the language with the design decisions the website author took. Just HTML+CSS is enough to make all the badly designed sites you want.