[steadicat]

Not to mention that UIs are visual, i.e. defining what is “correct” in terms a computer can understand is very hard. There are dozens of criterias (colors, spacing, borders, shadows, fonts, behavior, etc.), and often what is accetable is fuzzy and sometimes even subjective.

Take spacing: if something moves 200px it's probably broken. But a couple pixels off are usually fine. Unless those 2px are the vertical position of an icon inside a button or two things that need to align. Where do you draw the line? Humans are pretty good at spotting when things look off just by looking at a screen. Computers, not so much (yet).