[avaer]

Number one rule is don't start from scratch.

If you have something you already like and code is available, clone it and point the agent to the code. If not, bootstrap some code from screenshots or iteration.

Once you have something that works, your agent will be pretty good at being consistent with whatever you're going for and UI will be a "solved problem" from then on. Just point it to your reference code, and you can build up a component collection for the next thing if you like.

As a distant second, becoming familiar with design terminology allows you to steer better. Fold, hero, inline, flow, things like that. You don't need to know the code but if you can explain what it should look like you can complain to the LLM more efficiently.

Also, the model matters. I've found Opus 4.6 to be the best for web UI, but it probably matters what you're doing so experiment with your knobs a bit.