When you consider UX, accessibility, progressive enhancement, cross browser and cross device support - there’s plenty to know. And unlike programming, it doesn’t all flow logically. Some stuff is subjective, other stuff requires specific knowledge of browser quirks etc.
The only site that helped me gain some _understanding_ of CSS (as opposed to knowledge, tricks and hacks) was this one http://learnlayout.com
It’s a bit dated now - Flexbox is mentioned at the end and CSS Grid is entirely absent - but still important if you’re supporting older browsers.
But CSS has never “clicked” for me. For a start the whole idea of cascade is based on inheritance and I generally prefer composition. I like designing UIs and it’s quite a satisfying challenge to match a wireframe to the pixel. But overall, I’d rather be writing application code.
If users could just read JSON everything would be a lot easier...