Easing

Easing is the shape of time inside an animation. Entering and exiting elements use ease-out or a strong custom curve, so the movement lands the moment the eye cares about. Using ease-in on a UI element is a mistake: it delays the instant the user is waiting for. CSS built-ins like linear and the default ease are too weak for interface work — reach for a custom cubic-bezier instead. A curve that starts fast and settles slow reads as responsive; one that starts slow reads as lag.