Here’s a comprehensive and great-looking CSS reference featuring lots of examples.
While these days I normally reach for MDN Web Docs to keep me right on any given CSS feature, I’ve been thinking that with the recent explosion of new CSS features and syntax it’d be great to do a refresher course by working my way through a structured list of features—just like I did with Eric Meyer’s CSS: The Definitive Guide back in the day—and Codrops’ reference could be the perfect place.
In a similar vein, the CSS-Tricks Almanac looks good and cssreference.io is worth bookmarking, too.