Good motion explains. It shows where something came from, where it went and what just changed, so people never have to wonder whether that panel closed or vanished. If an animation can't explain anything, it's decoration, and decoration gets old by the third time you see it.
These essays are about making motion earn its place. Choosing durations and easing that feel quick without feeling abrupt. Letting things grow from whatever opened them. Using springs that can change direction halfway through. And remembering that motion runs on someone's device, and that some people have asked to see less of it. Even then, they still need to know what moved.
Every animation should explain something. If it can't, it's decoration.
Two numbers decide how motion feels. Most interfaces get both slightly wrong.
Things should arrive with some care and leave quickly.
A popover should open from the button that opened it, not from its own center.
Fixed durations break when people change their mind. Springs don't.
When an element changes place or shape, animate the change so people can follow it.
Hitting a hard stop feels broken. A little resistance feels physical.
People who ask for less motion still need to know what changed.
Some properties animate for free. Others make the browser redo the page every frame.