[tjoekbezoer]

I'd say a good start would be:

* Stacking context [1] — how individual layers interact with eachother to form a layout.

* Box model [2] — the most important properties that give an individual element its dimensions.

[1]: https://tiffanybbrown.com/2015/09/css-stacking-contexts-wtf/

[2]: https://css-tricks.com/the-css-box-model/