When I speak with some developers they often describe CSS as if they are fighting a battle. They managed to outmanoeuvre CSS and come out on top.
CSS can be challenging when your just starting but it's even difficult for long time programmers. Read about what I think some challenges are here.
The biggest challenge I found when learning was positioning elements. It's gotten much simpler with the advent of Flexbox and Grid and I'm happy about that. The mix of
position: relative and
float: left spanning multiple elements was very hard to get right as a beginner.
What do you think the biggest mistake new developers make when learning CSS?