“CSS”
30 Seconds of CSS
A curated collection of useful CSS snippets you can understand in 30 seconds or less.
Google HTML/CSS Style Guide
Utility OpenType Simple, CSS utility classes for advanced typographic features.
Simple, CSS utility classes for advanced typographic features. Falls back gracefully through feature queries.
Custom properties for breakpoint debugging
— That Emil is Emil Björklund
Learn Flexbox for free
– Scrimba.com
CSS: The Definitive Guide, 4th Edition
An A List Apart Article
simpl.info
The Ultimate Flexbox Cheat Sheet
Stylelint
A mighty, modern CSS linter and fixer that helps you avoid errors and enforce consistent conventions in your stylesheets.