Codedraft

Codedraft allows you to learn CSS at your own pace. Start with basics, then move to more advanced concepts.
Codedraft divides the courses into small bits of information so that you can study at your own pace. Occasionally you will get questions about what you have just studied.
The main attraction of Codedraft is the possibility to test yourself in real time by pairing the mobile application with the web application. This gives you access to dozens of free practical exercises that you can do to improve your skills and specialize in any aspect you want.
Codedraft offers two courses at the moment - CSS Basics and Advanced CSS. Each course has 13 and 7 sections respectively, and each section comes with its own practical exercises. And all of this for free, no account required.