UX Challenges - Done reading about UX? Start doing it.
Befront - Learn UX/UI design skills in practice with bite-sized tasks
Uxcel - The easiest way to learn UX/UI design
Frontend Mentor - Supercharge your front-end skills by building in a real-life workflow. Solve real-world HTML, CSS and JavaScript challenges whilst working to professional quality designs.
uxtoast: Learn UX Design - Become familiar with design laws and learn how to use them.
Divize - Learn, Write, Master: HTML/CSS Through Real UI Challenges