web-animations/timing-model/animations/reverse-running-animation-ref.html

<!DOCTYPE html>
<title>Reference for reverse running animation</title>
<style>
  #box {
    background: green;
    height: 40px;
    width: 40px;
  }
</style>
<body>
  <div id="box"></div>
  <p id="notes">
    This test animates the box color from green to red and reverses the play
    direction shortly after the midpoint. If the box remains red, the test
    failed.
  </p>
</body>