"Stole" it from Bulma's source code. It adds a loading spinner to buttons to give users feed back.

loading-spinner.css

// Hat-tip to bulma
.loader {
  color: transparent !important;
  pointer-events: none;
  position: relative;
}

.loader:after {
  animation: spinAround 500ms infinite linear;
  border:  2px solid #fff;
  border-radius: 50%;
  border-right-color: transparent;
  // border-top-color: transparent;
  content: "";
  display: block;
  width: 1em;
  height: 1em;
  position: relative;
  position: absolute;
  left: calc(50% - (1em / 2));
  top: calc(50% - (1em / 2));
  position: absolute !important;
}
@keyframes spinAround {
  from {
    -webkit-transform: rotate(0deg);
    transform: rotate(0deg);
  }
  to {
    -webkit-transform: rotate(359deg);
    transform: rotate(359deg);
  }
}