I'm sadly not surprised to hear that, same for another poster who had the displeasure of working with a codebase with !important sprinkled everywhere.
Truth is I've been spoiled on that front because I had the luck to work for many year in a company where the CTO was a very talented dude who lived and breathed HTML/CSS since the mid-2000's, and who was quite thoughtful about that kind of stuff, so I had a first-hand experience of a good codebase, mostly by avoiding external dependencies, using the standard browser behavior, and frequently consulting the MDN.
This is why it's a bit disheartening to see so many front-end framework working against the plateform, even even marketing against it, while CSS and browser tooling his so much more enjoyable and powerful than before.
True, but the thing is that it wasn't really "complicated" CSS we used, but simply a good grasp on the fundamental of CSS, which is a overlooked part of web development.
I believe it's mostly due to the fact that CSS, just like SQL, is one of those technology approachable enough to fiddle with it and get somewhat far, while not really learning it, and thus overlooking the fundamentals.
> None of the frameworks I know of do that.
I would squarely put into this category all the CSS-in-JS frameworks, such as 'styled components'. Also everything that markets the 'cascading' part of CSS as being a mistake.
Also, all the frameworks that compiles CSS without generating the CSS sourcemaps, making the debugging experience way less efficient since all the applied CSS rules shows under '<style>' (eg: 'styled-components' again, who discarded it as being 'cosmetic' in the related Github issue).
I've yet to see a single project that doesn't fail on all three.