Shay Howe’s Learn to Code HTML & CSS Covering the box model, positioning, typography, forms, tables, and organising your code.
Learn Layout CSS fundamentals of layout. Assumes you already know a little bit about how to write CSS, but want to know more about layout specifically.
Learn CSS Layout (the pedantic way) A more technical take on CSS looking at positioning, the box model, flexbox, and grids.
Web Platforms learning guides for HTML and CSS A mix of definitions and examples. Covering a wide range of practical things, plus some best practice advice.