A co-worker asked for a recommendation for a good CSS course for them to learn further. They are not a total beginner and can do fairly complex styling and the like, however, they have been 100% self taught and so have picked up things that are just the wrong way of doing it.

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?