I have yet to find a frontender (and I have worked with 100s over the past 30 years) that can do any significant css without trying it out in a browser; if there is something off or needs to change on the frontend, very often the most experienced senior frontender can guess (and often a little bit right) what it is but to actually fix it they have to try a few times.
To me (someone who likes logic) that is a sign it’s all not very good. I can write a bunch of code on paper in several programming languages and even non web frontend frameworks and it will work after I type it in first time ; when it comes to web frontend, it has to be tried out to know it works as it likely will have issues (unless it has no css layouting of course or absolutely trivial prototype stuff).
It seems like you're not comparing apples to apples. Investigating someone else's code to alter it is always going to be harder than writing new code, even on paper in several languages.
I do think that CSS has enough footguns to say that arbitrary CSS is not very good; well-written CSS is a very small subset of CSS in general. It joins many other languages in that regard-- C, C++, Javascript, PHP, Bash, the list goes on and on. As in all of those languages and more, with some basic familiarity it's not hard to sling some fresh code together and have it do what you want. Making it scale maintainably is the hard part.
(For my 2 cents on the matter, the way to write Good CSS(tm) is using CSS modules to limit scope, using flexbox or grid for all layout, and being rigorous about creating stacking contexts when you need to modify z order. Application of these three rules has made my life as a senior frontend engineer generally quite easy.)
To me (someone who likes logic) that is a sign it’s all not very good. I can write a bunch of code on paper in several programming languages and even non web frontend frameworks and it will work after I type it in first time ; when it comes to web frontend, it has to be tried out to know it works as it likely will have issues (unless it has no css layouting of course or absolutely trivial prototype stuff).