css/css-animations/animation-delay-010.html

<!doctype html>
<html class="reftest-wait">
<meta charset="utf-8">
<title>CSS Animations Test: animation-delay - liveness with
  animationstart</title>
<link rel="author" title="Brian Birtles" href="mailto:bbirtles@mozilla.com">
<link rel="help" href="https://drafts.csswg.org/css-animations-1/#animations">
<meta name="assert" content="Check that extending the animation-delay triggers
an animationstart event">
<meta name="flags" content="dom">
<link rel="match" href="animation-common-ref.html">
<style>
@keyframes all-orange {
  from { background-color: orange }
  to   { background-color: orange }
}
div {
  width: 100px;
  height: 100px;
  background-color: red;
}
</style>
<div></div>
<script>
// Set up animation with a negative delay such that it finishes very soon
var div = document.querySelector('div');
div.style.animation = 'all-orange 1000s -999.99s';

// Wait for the animation to finish
div.addEventListener('animationend', function() {
  // Set up an animationstart event handler to change the background color
  div.addEventListener('animationstart', function() {
    div.style.animation = 'none';
    div.style.backgroundColor = 'green';
  });

  // Then extend the delay so that the animation restarts
  div.style.animationDelay = '0s';

  // Wait a frame to allow the event handler to run
  window.requestAnimationFrame(function() {
    document.documentElement.removeAttribute('class');
  });
});
</script>
</html>