A long course covering how to plan and write CSS for real projects, with the Principles module free to read.

A Piccalilli course by Andy Bell. It has 51 lessons in 8 modules, built around one project: a home page and pattern library for a fictional barista academy.
It goes from principles (fluid type and space, layout, CSS organisation, progressive enhancement) through planning, prototyping, building core components and composing pages. Lessons are written with demos, interactive parts and videos, and take about 12 to 20 hours.
You need basic HTML and CSS. Eight lessons are free to read, and buying it includes lifetime updates, a certificate and a Discord community.
@layergrid