Getting Deep Into Shadows | CSS-Tricks - CSS-Tricks
Box Shadow SCSS Mixin - Codepad
Bootstrap on Twitter: "We'll be bringing CSS variables to more components in each minor v5 release to make customization easier and even reduce file size in some cases. Consider this before and
An Introduction to CSS Preprocessing - Speaker Deck
How to Make Pixel Art With Box Shadows - Webtips
The master guide to smooth, realistic shadows in CSS - DEV Community