Most intermediate courses I've found focus on CSS frameworks or precompilers and the like, he doesn't want this as most of their work is landing pages etc so the CSS course needs to focus on plain CSS with HTML.
Any recommendations?
I would also recommend the “thinking of ways to solve…” gui challenges [3] YouTube videos and web.dev articles by Adam Argyle.
Others have mentioned Josh W Comeau who definitely puts out good articles as well.
In the end, as with everything programming related it comes down to actually building stuff and getting your hands dirty. CSS is really something you can only get better at by running into real world issues and discovering its quirks.
[2] https://www.defensivecss.dev
[3] https://youtube.com/playlist?list=PLNYkxOF6rcIAaV1wwI9540OC_...