<!doctype html>
<link rel="author" href="mailto:emilio@crisal.io" title="Emilio Cobos Álvarez">
<link rel="author" href="https://mozilla.org" title="Mozilla">
<style>
.ref { width: 14em; height: 1em; margin-top: 0; margin-bottom: 0;}
</style>
<body>
<div id="ref"></div>
<script>
const TEST_CASES = [
  ["blue", "red"],
  ["blue", "green"],
  ["rgb(255, 0, 0, .2)", "red"],
  ["blue", "red", 0.9],
  ["blue", "red", 0],
  ["currentColor", "white"],
  ["currentColor", "rgba(0, 0, 0, .5)"],
];

const refElement = document.getElementById("ref");
const refStyle = getComputedStyle(refElement);

let animation = null;

for (let [from, to, animationProgress] of TEST_CASES) {
  const animationProgressExplicit = animationProgress !== undefined;
  animationProgress = animationProgressExplicit ? animationProgress : 0.5;
  // Set up the ref.
  if (animation) {
    animation.cancel();
  }
  animation = refElement.animate({
    backgroundColor: [from, to],
  }, { duration: 1000 });
  animation.pause();
  animation.currentTime = 1000 * animationProgress;

  let results = animationProgress == 0.5 ? 4 : 3;

  for (let i = 0; i < results; ++i) {
    let element = document.createElement("div")
    element.classList.add('ref')
    element.style.backgroundColor = refStyle.backgroundColor;
    document.body.appendChild(element)
  }
}

refElement.remove()
</script>
</body>