 const target = createDiv(t);
    const anim   = target.animate(null, { duration: 1000,
                                          fill: 'forwards',
                                          easing: params.easing });

    for (const sampleTime of [0, 250, 500, 750, 1000]) {
      anim.currentTime = sampleTime;
      const portion = sampleTime / anim.effect.getComputedTiming().duration;
      const expectedProgress = params.easingFunction(portion);
      assert_approx_equals(anim.effect.getComputedTiming().pr