CSS only truly exists in a browser. As soon as we start writing CSS outside of the browser, we rely on guesses and memorization and an intimate understanding of the rules. A text editor will never be able to provide as much information as a browser can.
CSS is frustrating because you have to actually think of a website like a website and not an app. That mental model is what everyone finds so viscerally upsetting. And so engineers do what feels best to them; they try to make websites work like apps, like desktop software designed in the early naughts. Something that can be controlled.
Why do people respect JavaScript or other languages enough to learn them inside-out, and yet constantly dunk on CSS?
The headline begs the question, but Robin makes this very insightful observation in the article itself:
I reckon the biggest issue that engineers face — and the reason why they find it all so dang frustrating — is that CSS forces you to face the webishness of the web. Things require fallbacks. You need to take different devices into consideration, and all the different ways of seeing a website: mobile, desktop, no mouse, no keyboard, etc. Sure, you have to deal with that when writing JavaScript, too, but it’s easier to ignore. You can’t ignore your the layout of your site being completely broken on a phone.