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

body {
  background-color: #555;
}

@keyframes move {
  0% {
    margin-left: 0;
  }

  50% {
    margin-left: 260px;
  }

  100% {
    margin-left: 0;
  }
}

.wrap {
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  width: 300px;
  margin: auto;
  height: 40px;
  background-color: #444;
  border-radius: 40px;
}

.loader {
  width: 40px;
  height: 40px;
  border-radius: 40px;
  position: relative;
  background-color: #333;
  animation: move 1.5s infinite;
}