Earlier quoted context omitted.
Is there strong scientific evidence that when one of these is slightly violated, it will negatively impact the user?
The short answer is no, slight violations of these ideas don’t tend to cause the sky to fall. Indeed, the list of examples given by nikatwork was unfortunate, because some of them are well established with a decent evidence base, but a few are basically just nonsense that has unfortunately somehow become dogma. For example, a few studies have been performed under controlled conditions to try to determine how typograp…
Of course, the other side to design is "style", which cannot really be quantified, merely subjectively judged against current trends, the target audience and "taste".