.double_box {
  position: relative;
  border: 1px solid #0f0;
  width: 200px;
  height: 100px;
  cursor: pointer;

  &::before,
  &::after {
    content: "";
    position: absolute;
    width: 20px;
    height: 20px;
    transition: 0.2s ease-in-out;
  }

  &::before {
    top: -5px;
    left: -5px;
    border-top: 1px solid #0f0;
    border-left: 1px solid #0f0;
  }

  &::after {
    right: -5px;
    bottom: -5px;
    border-bottom: 1px solid #0f0;
    border-right: 1px solid #0f0;
  }

  &:hover::before,
  &:hover::after {
    width: calc(100% + 9px);
    height: calc(100% + 9px);
  }
}

.double_box_demo {
  position: relative;
  border: 1px solid #0f0;
  width: 200px;
  height: 100px;
  cursor: pointer;

  &::before,
  &::after {
    content: "";
    position: absolute;
    width: 20px;
    height: 20px;
    transition: 1s ease-in-out;
  }

  &::before {
    top: -5px;
    left: -5px;
    border: 1px solid #0f0;
  }

  &::after {
    right: -5px;
    bottom: -5px;
    border: 1px solid #0f0;
  }

  &:hover::before,
  &:hover::after {
    width: calc(100% + 9px);
    height: calc(100% + 9px);
  }
}