By UI I meant that they're capable of creating beautiful user interfaces throught strong command of CSS on their own. This implies that they have a good eye for what looks good vs not. I guess this what we commonly refer to as graphic design but is implemented for real instead of living in a figma file.

They also have good design sense (UX). So they know what makes an intuitive UI and where to put things. They also can fill in on the designer's blind spots when doing the implementation of a design.