<!DOCTYPE html>
<html class="reftest-wait">
<style>
#target {
  animation: red 100s;
}
@keyframes red {
  0% { background-color: red; }
  100% { background-color: red; }
}
div {
  background-color: green;
  width: 100px;
  height: 100px;
}
</style>
<div id="target"></div>
<script>
document.addEventListener('MozReftestInvalidate', () => {
  requestAnimationFrame(() => {
    document.styleSheets[0].cssRules[0].style.animationName = 'non-existent';
    document.documentElement.classList.remove('reftest-wait');
  });
}, false);
</script>