A free, self-paced course from the box model through layout, colour, functions and animation, with demos and podcast episodes.

A free CSS course and reference on web.dev, written by members of the Chrome team and outside experts. It has 38 modules, from the box model, selectors, the cascade and specificity through flexbox, grid and logical properties.
Newer modules cover nesting, anchor positioning, popover and dialog, view transitions for single-page apps and container queries. Each module is an article with demos, many with a short knowledge check.
A final quiz earns a Learn CSS badge, and modules link to matching episodes of The CSS Podcast.
gridclamp()@layer