Long, interactive articles that explain how CSS really works.

This is the CSS section of Josh W. Comeau's blog, with about 30 articles. Many are interactive, such as "An Interactive Guide to CSS Grid" and "An Interactive Guide to Flexbox".
Articles cover layout algorithms, subgrid, container queries, :has(), anchor positioning, @starting-style, colour formats, shadows and a modern CSS reset. The articles and newsletter are free; he also sells courses like CSS for JavaScript Developers.