.shape {
  position: absolute;
  height: 100px;
  width: 100px;
  border-radius: 50px;
  background-color: blue;
  animation-name: opacity;
  animation-duration: 2s;
  animation-iteration-count: infinite;
  margin: 100px;
}

@keyframes opacity {
  0% {
    opacity: 0%
  }
  50% {
    opacity: 100%
  }
  75%% {
    opacity: 100%
  }
  100% {
    opacity: 0%
  }
}