And here is their similar guide for Flexbox which I actually have bookmarked and attached to a keybinding in my editor since it's so goddamn useful!

https://css-tricks.com/snippets/css/a-guide-to-flexbox/

http://cssgridgarden.com/ is a gentle intro to grid in the form of a game.

http://flexboxfroggy.com/ is equally fun and instructive.