on: 1000, fill: 'both' });

  anim.currentTime = 0;
  assert_equals(anim.effect.getComputedTiming().progress, -2);
  // Input < 0 ==> output = 0.
  assert_equals(getComputedStyle(target).opacity, "0");
  anim.currentTime = 500;
  assert_equals(anim.effect.getComputedTiming().progress, 0);
  // Input = 0 & before flag = false ==> output = 1.
  assert_equals(getComputedStyle(target).opacity, "1");
  anim.currentTime = 1000;
  // Input > 0 & before flag = false ==> output = 1.
  assert_equals(anim.effect.getCo