There's also this site which allows easy switching/previewing of classless CSS themes:

https://www.cssbed.com/

I like that resource, but it's missing an important one: github-markdown-css[1]. Given that most devs these days read a lot of Github markdown, it seems like a reasonable starting place for a css system.

1 - https://github.com/sindresorhus/github-markdown-css

Reminds me of https://csszengarden.com/

I've been waiting for someone to mention this just to check how many older designers/devs are here.

Tufte looks very elegant

Tufte frequently makes the rounds here — most recently 4 days ago: https://news.ycombinator.com/item?id=45119103

Very clean especially with the Roboto condensed example in the comments of the shared Tufte link.

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See also https://dohliam.github.io/dropin-minimal-css/ which has quite a few more (including github-markdown-css mentioned in a sibling comment).

Nice.

There are some nice one there, surprised many weren’t mobile-first or mobile-friendly.

Pico looked nice on mobile though.