keyframes spin 100 transform rotate 360deg spinner width 30px height 3

@keyframes spin {
  100% {
   transform: rotate(360deg);
  }
}
.spinner {
  width: 30px;
  height: 30px;
  border: 6px solid #000;
  border-left-color: #333;
  border-right-color: transparent;
  border-bottom-color: #555;
  border-radius: 100%;
  animation: spin 600ms infinite linear;
}

<div class="spinner"></div>