No JavaScript. The pendulum below is one <div> rotated by a keyframe
animation timed to a soft cubic-bezier so it spends most of its time at
the apex — like a real pendulum decelerating against gravity.
Why it works
The key is the cubic-bezier curve (0.5, 0, 0.5, 1). With that shape the
animation slows down at both ends — a passable approximation of pendulum
physics for casual viewing. A real pendulum’s motion is cos(t); this is
“close enough that you don’t notice.”
prefers-reduced-motion cancels the swing for users who don’t want it.