s

A second is the larger time unit, and 1s equals 1000ms. Time values need their unit even when they are zero: 0s is valid in transition-duration, a bare 0 is not. The sign matters as well. A duration cannot be negative, but a delay can, and animation-delay: -2s starts the animation as if it had begun two seconds ago. On a 4s animation that is the halfway point.

Seconds suit motion that people count: a slideshow slide that stays for 5s, a shimmer that loops every 2s. Duration has an accessibility side too. WCAG 2.2 success criterion 2.2.2 (Level A) asks for a way to pause, stop or hide movement that starts automatically, lasts more than five seconds and sits next to other content. What counts is how long the movement runs, not the number in the declaration. A loop of 2s that repeats forever runs past five seconds and falls under it, and a single 300ms transition does not.

For interface feedback such as hover and focus changes, the numbers get small enough that ms is easier to read.