$l: 16em;
.wrapper {
  position: absolute;
  top: 50%; left: 50%;
  width: $l; height: $l;
  transform: translate(-50%, -50%) rotate(45deg);
}

.square {
  float: left;
  width: 50%; height: 50%;
  animation: rot 2s ease-out infinite;
  &:empty { background: black; }
}
@keyframes rot { to { transform: rotate(90deg); } }