How will an LLM "take greater care with typography" if it can't see the page it is creating? How will it "improve" leading if you need a human to see that there's too much distance between lines or too little?

With playwright, it can see what it is creating. Unsurprisingly, it works much better if you hook it up to a browser.

Because humans have already annotated diagrams and examples of what ‘too much’ and ‘too little’ look like, and these have been incorporated into the model. It tries to reproduce the content that is associated with humans indicating that they are taking greater care, and that content has the ‘not too much / not too little’ judgement already baked into it.

I'm assuming this is in the context of an agent that can see what it's doing. And I wouldn't assume humans have a monopoly on judging leading.