$timing: 1000ms*sqrt(2) infinite linear;

html {
  height: 100%;
  width: 100%;
}

body {
  position: relative;
  height: 100%;
  width: 100%;
  background: black;
  color: white;
  padding: 50px;
  
}

section {
  position: absolute;
  top: 50%;
  left: 50%;
  margin: -500px/sqrt(2);
  width: 1000px/sqrt(2);
  height: 1000px/sqrt(2);
  overflow: hidden;
  background: black;

}

div {
  box-sizing: border-box;
  position: absolute;
  top: 50%;
  left: 50%;
  margin: -50%/sqrt(2);
  width: 100%/sqrt(2);
  height: 100%/sqrt(2); 
  border-radius: 100%/sqrt(2)/4;
  transform: rotate(45deg);
  animation: rotate $timing;
  background: #fff;
  backface-visibility: hidden;
  overflow: visible;


  &.a {
    animation: zoom $timing;
    transform: scale(sqrt(2)*2);
  }
  
  &.e {
    background: #000;
  }
  
  &.tb {
    animation: tb $timing;
  }
}

@keyframes zoom {
  0% {
    transform:  rotate(0deg) scale(sqrt(2)*2);
  }
  100% {
    transform:  rotate(360deg) scale(sqrt(2));
  }
}

@keyframes tb {
  100% {
    opacity: 0;
  }
}