Roadmap.sh This site has very detailed pathways to learn Frontend, Backend, and other developer pathways.
FrontEndPractice A site with several projects where you can practice your frontend knowledge.
UserInYerFace Deliberately annoying and yet a fun game to play and learn frontend issues.
Web Page Anatomy Learn the anatomy of web pages in 14 basic elements by Tubik Studio
Fundamental CSS Make your life easier by learning the most fundamental concepts of CSS first