I’m a huge fan of the flexbox spec in CSS. Simply making vertical centering simple is worth the price of admission. That said, CSS Grid may be a bigger deal. In short, it makes CSS layouts a real practice in a way that has never really been the case before. In slightly less short, watch this 3+ minute video of Jen Simmons on why she loves CSS Grid. In full, watch this talk by Rachel Andrew, the foremost CSS Grid evangelist.
Assuming you’re on board with Grid (Should I still be using uppercase? Not sure.), it’s time to learn how to use it. I first played with it using the cheat sheet on CSS Tricks, but the game by Thomas Park is opening all the doors for me. It’s simple, incremental, and seems pretty comprehensive. The “levels” are so small that you can do a few at a time and keep the tab open for later. I really encourage you to make use of this as I have.