If you want to learn CSS and Web Development in general, there is no better resource than Free Code Camp. Endless hours of lessons and projects and a path to certification and a new career. All of it for free. This site represents the future of education to me. I have been a member since May 2017.
A endless resource on all things Web Development. They will have definitions and examples of every CSS property you can think of. Usually, they do a decent job of letting you know the basic technical information about a given topic. They don't go into detail about how to use it and there examples are usually generic. A good starting point for grasping general concepts
Another huge reference! I tend to like these guys better than MDN. This is mostly due to there examples. They always have a working example and you can jump right into their code editor and play around with whatever the subject is. They also give basic, generic definitions. The site is well organized and you can follow many different learning paths under the Web Development umbrella
Awesome collection of articles that gets constantly updated. This is the place to go search around in to learn the tricks of the trade. They have been around for a few years so chances are you can find what you are looking for here
If you have a specific question you can start by doing a search here. If anyone else had a similar question you will find your answer in the question archives. If a search doesn't work, you can post your question for the community to answer. Chances are you will get a response within a few hours. Just make sure you do your research first, having duplicate threads is frowned upon in the Stack Overflow community.
Looking for the world's most popular front end framework. Check out Bootstrap if you haven't already. They make applying CSS a lot easier through the use of their pre defined classes. Knowledge of this framework is pretty much universal at this point. Everyone has used Bootstrap at some point in their coding lives
I had better not forget these guys. This site is built on top of Foundation. They have a nice CSS framework with a cool style that is noticably different from Bootstrap. A lot of the functionality is pretty similar and for me it comes down to wanting to try new things. I am still undecided on my preferred front end CSS framework