Box model + flex box + the display modes + "positioning" (stacking contexts) are typically the most confusing parts of CSS, I feel.

Add maybe margin and padding, and how they interact (margin collapse, no margin collapse in flex or grid context, box model again, auto values).

But then, that's basically all the hard conceptual parts. But they are mostly intuitive and well-designed! I know I'm alone with this opinion.

Naming + global scope is the rest of the problem, but even without this, here is where the problem of creating design system shows up, too. Harder than CSS probably :)