I think coding assistants aren't great at UI/UX yet because they can't see, their understanding of left/right/lighter/darker is guessed from textual descriptions that accompanied CSS tutorials but they are never actually imagining the looks of what they are working with. I had Cursor repeatedly fix and mess up a CSS grid, over and over again, until I switched to HTML table so that browser would handle layout. Once switched from visuals ("leftmost") to semantics ("first cell in a row") the agent immediately started getting tasks done right.
I guess keep them on backend/library tasks for now. I am sure the companies are already working on getting a snapshot of a browser page and feeding it back into multimodal model so it can comprehend what "looking" means.