body::before {
  content: "";
  position: fixed;  /* 固定定位 */
  top: -10px;
  left: 0;
  width: 100%;
  height: 10px;
  overflow:hidden;
  box-shadow: 0px 0 10px rgba(0, 0, 0, 0.8);
  z-index: 999;  /* z-index 值设置大点，防止被覆盖 */
}