I’ve also been writing CSS for about 15 years, and I have endless complaints. I think the provisions for maintaining and composing styles are just really lacking out of the box. It’s better with the recent addition of calc and var, but even those feel strange, underpowered and bug-prone. This is aside from complaints about the many overlapping, poorly-designed layout systems that interact in baffling ways, and the mindless profusion of syntax variations.
So are your "endless complaints" about web designing documents or applications? (curious to update my experience database with your response)
I think CSS was designed for documents and not apps. And almost all the real difficulties I have with CSS (and all the ones I hear from others) is with application development, not web design per se.
CSS is my favorite coding "language", but I have always been an artist & coder.