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                <button id="loopControl">play/pause</button>


                body {
  display: flex;
  min-height: 100vh;

button {
  margin: auto;


 * Given a value for hue in degrees, generate a set of steps to
 * transition the saturation and lightness values for that hue.
 * THIS IS COMPLETELY ARBITRARY, need to just eyeball it and add/remove
 * definitions until it kinda looks ok. Can also add timings for each
 * in attempt to make it look nicer/smoother.
const slTransition = (hue) => [
  { backgroundColor: `hsl(${hue}, 100%, 50%)`},
  { backgroundColor: `hsl(${hue}, 50%, 100%)`},
  { backgroundColor: `hsl(${hue}, 0%, 50%)`},
  { backgroundColor: `hsl(${hue}, 50%, 100%)`},
  { backgroundColor: `hsl(${hue}, 100%, 50%)`},

 * The animations API expects an array of transition objects, so
 * produce and array of the hue values [numbers] 0 to 359, and for
 * each of those, map using the `slTransition` function to give
 * all transitions for all hues.
const transitions = Array.from({length: 360 }, (_, i) => i).flatMap(slTransition);

 * There are 360 hue steps, and each step has however many steps are defined
 * in the `slTransition` function. If all of those steps have the same timing,
 * then can just multiply by some arbitrary number of milliseconds for simplicity.
const STEP_DURATION = 500;
const timing = {
  duration: 360 * 5 * STEP_DURATION,
  iterations: Infinity

// Keep a reference to the animation so can pause/play
const loop = document.body.animate(transitions, timing);
// It will always start automatically, so pause it immediately

const loopControl = document.querySelector("#loopControl");

loopControl.addEventListener("click", () => {
  if (loop.playState === "paused") {;
    loopControl.innerText = "pause";
  } else {
    loopControl.innerText = "play";