Edit in JSFiddle

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

.loader {
  width: 50px;
  height: 50px;
  border: 5px solid #f3f3f3;
  border-top: 5px solid #3498db;
  border-radius: 50%;
  animation: spin 1s infinite ease-in-out; /* linear */
  margin: 20px auto;
}

@keyframes spin {
  0% {
    transform: rotate(0deg);
  }
  100% {
    transform: rotate(360deg);
  }
}