Not the author, but thought it was pretty nice.

- https://github.com/DigitallyTailored/Classless.css

I know this isn't the first, I have this note from a previous HN discussion:

- https://github.com/andybrewer/mvp

- https://github.com/kognise/water.css

- https://github.com/xz/new.css

- https://github.com/vladocar/Basic.css

- https://github.com/oxalorg/sakura

- https://github.com/igoradamenko/awsm.css

- https://github.com/hawkz/gdcss

- https://github.com/emareg/classlesscss

- https://github.com/vincentdoerig/latex-css

- https://github.com/dbohdan/classless-css

- https://picocss.com/examples/preview/

- https://github.com/kevquirk/simple.css

Aggregate tool: https://dohliam.github.io/dropin-minimal-css/