Oldschool CSS was not much fun, but I never understood how this made it any better:

<div><div><div><div><div><div>Bootstrap</div></div></div></div></div></div>

I always forgot a row, had cols inside of cols, forgot how form-groups worked, or found other ways of messing up the whole layout.

Instead of complex CSS, there was now this new complex language entirely expressed through the nesting of layers upon layers of divs. It was like LISP's brackets, but more verbose.

That was the moment I realized that fullstack is bullshit, that there are intrinsic talent differences between frontend and backend devs, and that it's OK to focus on a narrower but deeper field.