.box{
    width: 100px;
    height: 100px;
    border-radius: 50%;
    box-shadow: 0 0 0 10000px -sharp000;
    background-color: transparent;
    animation: border_shrink 1s;
  }
  @keyframes border_shrink {
    0% {
      width: 100px;
      height: 100px;
    }
    100% {
      width: 0;
      height: 0;
    }
  }