I’m doing it on my personal site, tyleo.com.

This question comes up enough that I’m writing a guide on it now.

I don’t use any CSS framework except CSS modules. I was going to write the guide without module support and add a section about it near the bottom.

There’s some really simple patterns you can follow with built-in JS features to make this easier.

Have you tried looking at web apps? My experience as a software engineer working mostly on web uis has been with (s)css modules and only recently with css-in-js. When working on large, complex projects we’ve always steered away from atomic css solutions like tailwind with the long lists of class names being difficult to maintain, awkward syntax, and subpar theming being the main reasons for it. Personally I go for pandacss which imo strikes a nice balance between atomic styles, type safety, and ergonomics. Still stirs away from web standards though

Do you mean 100% external CSS, or just any external? Because I've definitely seen teams implement the design system as external CSS, then adjust at the component level within the code. I've done 100% external CSS on private app with a few hundred users, but nothing public. I currently have a side project that uses React for content management, and builds static HTML with external CSS via an SSG for publishing.

So... it happens. But I use SPAs for internal apps, not public web sites, so despite my knowing such things exist, I got no links to share.