Popmotion has a nice API for defining animations, and some actions include a property to loop the action:

tween({
  from: {opacity: 0},
  to: {opacity: 1},
  duration: 2000,
  // highlight-start
  loop: Infinity,
  // highlight-end
}).start(el.set)

This will replay the animation for… Infinity:

https://codepen.io/tyronetudehope/pen/dBWPwV

However, with more complex animations, we can make use of the Action's start method callbacks. For example, if we'd like to create an animation which fades something in, displays it for a few seconds, then fades out again, and repeat ad infinitum, we can wrap it in a function and which calls itself from the complete() function:

const animation = chain(
  // Fade in
  tween({
    from: {opacity: 0},
    to: {opacity: 1},
    duration: 500,
  }),
  // Pause
  delay(2000),
  // Fade out
  tween({
    from: {opacity: 1},
    to: {opacity: 0},
    duration: 500,
  }),
  // Delay fade in
  delay(500),
)

const runAnimation = () => {
  animation.start({
    update: el.set,
    // highlight-start
    complete: runAnimation,
    // highlight-end
  })
}

runAnimation()

https://codepen.io/tyronetudehope/pen/JQNdjy