Documentation is for when you need to know intricate details of exactly how and why something works. If you're just getting started with CSS, try the tutorials
- MDN - Mozilla Developer Network
- Web Platform
Validators and Linters
Validators ensure that you are conforming to the specification. Linters help you conform to best practices and should be configurable to your own set of practices.
Simple (usually printable) pages containing lots of concise usage information.