I don't know if it's CSS or me but I never "got" CSS. I feel like it's more about remembering lots of unrelated things with no real clear logic behind it. An IDE helps a bit. Maybe if it was presented as a series of functions instead of properties, that takes certains types of variables? At least you could group some knowledge this way. Like height would take an union type like Pixels | Rem | Em | Percents | etc

I think JS (ES5+) is mostly fine, a bit of legacy cruft but it's often overstated. I'd like it if we didn't need something as complex as Typescript to fully type it, but that may just be my biais for ML-style languages. Also, Reason/Rescript/Bucklescript/whatever spent its complexity budget on branding, which is a weird choice.

HTML I'm mostly fine with it, but I'd like to see what people could do by starting from the beginning with it.

I don't know anything about GUI programming outside of the web (and Win/Webdev) so my vision is really limited here.