"Pixel perfect" is about attention to detail and consistency. Margins, padding, or the combination of these inside other containers will stick out when they're not consistent.
Here's an example that I personally encountered: what if you have a <h1>Text</h1> and it has a certain left margin.
Then another heading except it has a nested button component (which internally comes with some padding). Then the "Text" in both aren't aligned from section to section and it is jarring.
Now that Cursor has moved towards a credits system, grok code fast is making the plan last while still being reasonable in inference time. GPT 5 and GPT 5 Codex actually moves my "amount remaining" bar in realtime while being incredibly slow.
My experience with 'sonic' during the stealth phase had it do stuff plenty fast, but the quality was slightly off target for some things. It did create tests and then iterate on those tests. The tests it wrote don't actually verify intended behavior. It only verified that mocks were called with the intended inputs while missing the larger picture of how it is used.
Here's an example that I personally encountered: what if you have a <h1>Text</h1> and it has a certain left margin. Then another heading except it has a nested button component (which internally comes with some padding). Then the "Text" in both aren't aligned from section to section and it is jarring.