Beginner
A Five Minute Guide to Typography (pierrickcalvez.com)
Codecademy (www.codecademy.com)
How to Write CSS That Works in Every Browser (hacks.mozilla.org)
Introduction to BEM (getbem.com)
Learn CSS (www.codecademy.com)
What are the Benefits of Using a CSS Framework? (css-tricks.com)
Marksheet (marksheet.io)
Treehouse - CSS Foundations (teamtreehouse.com)
Learn CSS Layout (learnlayout.com)
CSS Fundamentals (www.codecademy.com)
Organizing HTML & CSS (www.codecademy.com)
HTML Dog (htmldog.com)
Styling with CSS (www.codecademy.com)
A Beginner's Guide to HTML & CSS (learn.shayhowe.com)
Front End Interview Handbook (github.com)
Modern CSS Explained for Dinosaurs (medium.com)
CSS Reference (cssreference.io)
The CSS Box Model (www.codecademy.com)
Web Design in 4 Minutes (jgthms.com)
Intermediate
Scalable and Modular Architecture for CSS (smacss.com)
Advanced