Most sites animate element by element, decided in the moment a component is built. The result is technically fine and emotionally incoherent — nothing feels like it belongs to the same object.
We write motion rules the way we write typography rules: a small vocabulary of curves, durations and triggers that every interaction draws from. A button press and a page transition should feel like the same hand made them.
In practice that means defining "enter," "exit," "hover" and "scrub" behaviors once, early, before a single screen is designed — then holding every subsequent decision to that standard.