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                <div class="arrow"></div>


                html, body { height: 100%; }
body {
  position: relative;

.arrow {
  width: 0;
  height: 0;
  position: absolute;
  top: 50%;
  left: 50%;
   * Normally we can use this method, but since our element has a width and height of 0
   * it has no effect (translate is relative to the dimensions of the element).
  transform: translate3d("-50%, -50%, 0");
   * Instead, we will use negative margins to pull the element into the center.
  margin-top: -100px;
  margin-left: -100px;
   * Typically you would use negative of half the element’s width and height for the
   * margins, but because the borders = 100px on all sides, the total = 200px, so we
   * take half of that, which equals 100px.
  border-top:    100px solid #2ECC40;
  border-bottom: 100px solid #2ECC40;
  border-right:  100px solid transparent;
  border-left:   100px solid transparent;
   * This slides the bottom border’s width between 0 and 100px to show how it affects
   * the element.
  animation: slide 2s linear alternate infinite;
  transition: background-color .15s ease-in-out;
 * Let’s show the background of the element when we hover.
.arrow:hover { background: #FF4136; }

@keyframes slide {
  0% { border-bottom-width: 100px; }
  100% { border-bottom-width: 0; }