An extensive CSS reference with all the important properties and info to learn CSS from the basics
Extensive reference sporting clear explanations, examples, live demos (using Codrops’ Playground), CSS compatibility tables (provided by Can I use…), etc..
The fact that Sara Soueidan wrote a lot of these articles gives away that they’re very – very! – high quality.
Look up HTML5, CSS3, etc features, know if they are ready for use, and if so find out how you should use them – with polyfills, fallbacks or as they are.
Awesome resource, listing all newish stuff, along with the fallbacks/polyfills you’ll need to use.