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That's a nice site to learn. This is my goto reference (they even sell it as a poster).

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

Essential supplemental material: https://css-tricks.com/flex-grow-is-weird/

This is my favourite resource for learning about flexbox: <https://flexboxfroggy.com/>

And this for grid: <https://cssgridgarden.com/>

Theres also a flexbox tower defense site that was pretty neat iirc. Similar concept/way of learning by gamification.

I've learned flex using similiar game, some kind of tower defence I think, it's astounding how quickly I memorized these things compared to standard learning by reading

Agree. I think you're referring to this: http://www.flexboxdefense.com/

I often find this flexbox cheatsheet handy https://flexbox.malven.co/