<div id="loader">
  <div></div>
</div>

#loader {
  border: 1px solid black;
  width: 100px;
  height: 100px;
  margin: 40px auto;
  overflow: hidden;
}

#loader > div {
  background: black;
  width: 10px;
  height: 100px;
  transform-origin: top center;
  animation: load 2.5s infinite linear;
}

@keyframes load {
  0% {
    transform: rotate(0deg);
  }
  25% {
    transform: rotate(-90deg);
    margin-left: -5px;
    margin-top: 0;
    transform-origin: top center;
  }
  50% {
    margin-left: -5px;
    margin-top: 95px;
    transform-origin: top center;
    transform: rotate(-90deg);
  }
  75% {
    margin-left: 95px;
    margin-top: 0;
    transform-origin: bottom center;
    transform: rotate(0deg);
  }
  100% {
    transform: rotate(0deg);
    transform-origin: bottom center;
    margin-left: 0;
    margin-top: 0;
  }
}