An extensive CSS reference offering way more content than MDN
Can I use
Interactive browser support tables for CSS (and HTML5).
This article explains what the cascade is and how this affects you.
Specificity and inheritance
Understanding specificity and inheritance is important to master CSS. This article will help.
CSS Box Model
An article explaining the foundation of layout on the web.
The Lengths of CSS
Overview covering absolute and relative units.
Fun with Viewport Units
Imparts the basics and shows some nifty use-cases.
Basic CSS Selectors
An introducing to the very basic CSS selectors you need to know.
Advanced CSS Selectors
Level up your knowledge. From attribute selectors to CSS3 pseudo classes.
Learn how to use CSS selectors with this fun little game.
CSS Variables: Why Should You Care?
A short introduction to CSS variables.
Locally Scoped CSS Variables: What, How, and Why
Describes the advantages of locally scoped CSS variables.
Using CSS variables correctly
Patterns and anti-patterns for using CSS variables.
Everything you need to know about CSS Variables
In depth article going beyond the basics of CSS Variables using real-world examples.
Getting Reactive with CSS
Learn CSS Layout
Learn about CSS layout techniques in 5 chapters.
Jen Simmons video series about the new CSS Layout possibilities.
Laying Out The Future With Grid And Flexbox
Introduction of a new layout system encompassing Flexbox, CSS Grid and the Box Alignment Module.
In depth information about how to use (and clear) floats.
A closer look at a few little-known things related to the CSS positioning layout method.
Shows in which cases it makes sense to use the display property inline-block for layouting.
A Complete Guide to Flexbox
All you need to know about Flexbox on one page.
Play with Flexbox examples on CodePen.
A tower defense game in the browser to learn about Flexbox with fun.
Learn all the basics of Flexbox with a fun game involving frogs and lily pads.
Community-curated list of flexbox issues and cross-browser workarounds for them.
A training course driven by a storyline where you use Flexbox and a crossbow to hunt zombies.
What the Flexbox?
A simple, free 20 video course that will help you master CSS Flexbox!
A Complete Guide to Grid
All you need to know about CSS Grid Layout on one page.
Grid by Example
Besides examples of how to use Grid this site also has additional useful learning resources.
Designing with Grid
Talk about the new layout possibilities CSS Grid is offering.
Lovely game where you write CSS code to grow your carrot garden.
Community-curated list of Grid interop issues and workarounds for them.
Learn CSS grid layout by mastering an adventure game.
CSS Transitions and Transforms for Beginners
An introduction to to CSS transitions and CSS (2D) transforms.
All you need to know about CSS Transitions
Also addressing advanced topics from chaining and events to hardware acceleration and animation functions.
CSS 3D transforms
Multi page tutorial with examples like card flip and carousel effects.
CSS Animation for Beginners
Imparts the concepts of CSS animations with keyframes.
Nice little page demonstrating which CSS properties are animatable.