THIS. Material Design guidelines tried to drill these into deisgners' head. But man, their damn artsy skulls are thiiiiick...

There should be a "Shadow + Fill if it's tappable/clickable. F please!" rule though in every design school. Every time I see it not followed I get a fit of rage, that unfortunately I need to swallow and not fire the designer because I know he's otherwise good at UX too.

It's should something like a "don't f drive when you're drunk rule" rule... common sense to every-f-body!