The Odin Project
The Odin Project is a site based on the concept that learning should be free and accessible. The site offers training in all things HTML & CSS as well as JS, Ruby & Databases. The lessons implement CodePen examples to show the items in action. The site is sponsored by Thinkful, another learning platform to help people who want to get into IT based fields.
CSSReference.io
The CSSReference.io site (and the sister site HTMLReference.io) is filled with all of the information about each CSS property, how it works, the attributes available, etc. One thing I love about this site is how they include a lot of visuals to show the end result of the code. This is definitely a site I will come back to to learn and reference. This site was developed by Jeffry Thomas, who has 15 years of web development experience.
CSS Tricks
CSS Tricks is a site that I have known about for awhile and that I love to go back to time and again to look things up, or to just browse around to see new things to try in the CSS world. I originally found this page while searching for help on CSS grid and flex. I loved their thorough documentation and loved their printable guides to CSS grid & flex! I have those printed and refer to them often as I'm building out CSS. The site's original creator is Chris Coyer, who co-founded CodePen (which I also love!).