body::after {
  position: fixed;
  content: '';
  left: -1000px;
  top: -1000px;
  animation: shadow-preload 0.1s;
  animation-delay: 3s;
}

@keyframes shadow-preload {
  0% {
    background-image: url(https://cdn.dcloud.net.cn/img/shadow-grey.png);
  }
  100% {
    background-image: url(https://cdn.dcloud.net.cn/img/shadow-grey.png);
  }
}