It might help to consider how stable an arbitrary human would be at this task.

User interfaces are like pieces of artwork. You really don't know until it's finished, and even then it's not for everyone.

LLMs are most effective when working with human parsed representations of websites. They are terrible at doing the actual parsing.

They're also good at working on a relative basis. One png screenshot of the viewport is a difficult hill to climb. The difference between two screenshots is much more informative. Deltas usually tell a much bigger story than dead reckoning against the base world model.

This, but I'd say that there is an engineering perspective that you can apply. What the developer world is trying to discover here is how to encode taste concretely - often this is things like really mechanical design system things (space/padding/margins/relative sizes/color contrast/...).

The hard point is doing that well deterministically in a way that is token efficient and which can be iterated on. I suspect it's difficult to encode that into an AI model of what's good taste.