Писал ли анимацию

@keyframes fadeIn {
  from { opacity: 0; }
  to { opacity: 1; }
}
.element {
  animation: fadeIn 1s ease-in-out;
}

function animate(element, duration) {
  let start = performance.now();
  requestAnimationFrame(function animate(time) {
    let progress = (time - start) / duration;
    if (progress > 1) progress = 1;
    element.style.opacity = progress;
    if (progress < 1) {
      requestAnimationFrame(animate);
    }
  });
}