CSS: Cascading Style Sheets: Reference of CSS from the Mozilla Developer Network.
cssreference.io: A free visual guide to CSS.
Things I Wish I’d Known About CSS: A list of things that are important to understand about CSS.
CSS Zen Garden: The same page styled in really different ways.
Same HTML, Different CSS: different styling for the same HTML in components.
Normalize.css: A modern, HTML5-ready alternative to CSS resets.
Eric A. Meyer CSS Reset: A reset written by the author of CSS: The Definitive Guide, 4th Edition, published by O'Reilly.
Articles about display: flex.
A Complete Guide to Flexbox: a comprehensive guide to Flexbox ayout, with many illustrations.
CSS Flexible Box Layout: Documentation about Flexbox layout from the MDN.
Grid by Example: A list of examples of using display: grid.
The CSS Cascade: a very stylish explanation of the CSS cascade.
CSS: An Art, a Science, a Nightmare (Everything You Should Know): "This article is for the middleground of people who have had to touch CSS a few times here and there but ultimately don't feel like they know what they're doing, or struggle with making basic layouts." (by Tania Rascia).
scrollbar: an article explaning how to style scrollbars.
Bootstrap. A very popular and complete CSS framework.
Bulma. A smaller and modular CSS framework. Based on flexbox.
Pure. Modular, responsive, CSS framework from Yahoo engineers.
Tail Wind CSS. Low-level CSS framework. You want to customize CSS but do not want frameworks that do too much and you have to "fight" them to do what you want.
Milligram. Minimalist CSS framework (only 2kB gzipped).
Transparent Background Images: There is no CSS background-opacity, but you can fake it.
Meta Theme Color and Trickery.
Incomplete List of Mistakes in the Design of CSS: if you have issues with CSS, rest assured that even the designers know about many of them.