calc()

MDN: calc()

CSS Tricks: A Couple of Use Cases for Calc()

Smashing Mag: Getting Started With CSS calc()

Hardcore CSS calc()

CSS Custom Properties (Variables)

Smashing Mag: A Strategy Guide To CSS Custom Properties

MDN: Using CSS custom properties (variables)

CSS Tricks: What is the difference between CSS variables and preprocessor variables?

CSS Variables: Why Should You Care? (good JavaScript examples)

Performance of CSS Variables

CSS Tricks: Responsive Designs and CSS Custom Properties: Building a Flexible Grid System

Grid

How We Adopted CSS Grid at Scale

Improved CSS Grid Workflow with Sass

Super-powered layouts with CSS Variables + CSS Grid

Super-Powered Grid Components with CSS Custom Properties

Flexbox

CSS Tricks: Chris Coyer's Complete Guide to Flexbox

Smashing Magazine: Harnessing Flexbox for Today's Web Apps

Flexbox Grid Systems

Getting started with Flexbox grid systems

CSS Tricks: Responsive Designs and CSS Custom Properties: Building a Flexible Grid System

Flexbox and Grid Games

Flexbox Froggy

Flexbox Defense

Grid Garden