HTML

<div class="pswp__preloader__icn">
  <div class="pswp__preloader__cut">
    <div class="pswp__preloader__donut"></div>
  </div>
</div>

CSS

.pswp__preloader__icn {
  opacity:0.75;
  width: 24px;
  height: 24px;
  -webkit-animation: clockwise 500ms linear infinite;
  animation: clockwise 500ms linear infinite;
}
 
/* The idea of animating inner circle is based on Polymer loading indicator by Keanu Lee https://blog.keanulee.com/2014/10/20/the-tale-of-three-spinners.html */
.pswp__preloader__cut {
  position: relative;
  width: 12px;
  height: 24px;
  overflow: hidden;
  position: absolute;
  top: 0;
  left: 0;
}
 
.pswp__preloader__donut {
  box-sizing: border-box;
  width: 24px;
  height: 24px;
  border: 2px solid #000;
   border-radius: 50%;
  border-left-color: transparent;
  border-bottom-color: transparent;
  position: absolute;
  top: 0;
  left: 0;
  background: none;
  margin:0;
   -webkit-animation: donut-rotate 1000ms cubic-bezier(.4,0,.22,1) infinite;
  animation: donut-rotate 1000ms cubic-bezier(.4,0,.22,1) infinite;
}
 
@-webkit-keyframes clockwise {
  0% { -webkit-transform: rotate(0deg) }
  100% { -webkit-transform: rotate(360deg) }
}
@keyframes clockwise {
  0% { transform: rotate(0deg) }
  100% { transform: rotate(360deg) }
}
@-webkit-keyframes donut-rotate {
  0% { -webkit-transform: rotate(0) }
  50% { -webkit-transform: rotate(-140deg) }
  100% { -webkit-transform: rotate(0) }
}
@keyframes donut-rotate {
  0% { transform: rotate(0) }
  50% { transform: rotate(-140deg) }
  100% { transform: rotate(0) }
}
 

用純CSS實現加載中動畫效果