Same, Kotlin is a huge fail, if you can't get mobile friendly pages in 2020 it's a complete fail in my book.

At first I was going to say "I agree… in 2020 there's no reason not to have a mobile-friendly docs site… but in my experience with documentation sites, not many people visit from mobile devices". However, I just checked web.dev and developers.google.com/web and our mobile traffic is non-trivial.

As a hobbyist and self-taught programmer I (pre-COVID) spent a lot of time reading documentation on my phone while commuting home from work.

I'm very pleased when documentation works well on a mobile device.

What sites worked well for you? Anything in particular about how they worked well (or how other sites have not worked well)?

If have to go back and find examples, but there's not really much to it at all. A long as it's formatted nicely for reading on a small screen, that's a success.

Most recently I suppose would be the Rust book: https://doc.rust-lang.org/book/

It's a delight on mobile. I've noticed many documentation sites she'd to be based some publishing platform that probably includes a good response mobile design, so it's a feature you can get "for free", yet not every docs site uses such a platform or just hasn't implemented their own responsive behavior.

I'll update if I can remember one I read recently that didn't have a great mobile experience.

It's really a very low bar.

Oof, I didn't check it on mobile. That definitely counts for something for sure.