.центрирование-на-экране
{
    position: absolute;
    left: 50%;
    top: 50%;
    transform: translateX(-50%) translateY(-50%);
}

.фон-экрана
{
    background-color: white;
    position: fixed;
    left: 0;
    top: 0;
    right: 0;
    bottom: 0;
}

/* https://loading.io/css/ */
.lds-dual-ring
{
  display: inline-block;
  width: 80px;
  height: 80px;
}
.lds-dual-ring:after
{
  content: " ";
  display: block;
  width: 64px;
  height: 64px;
  margin: 8px;
  border-radius: 50%;
  border: 3px solid black;
  border-color: black transparent black transparent;
  animation: lds-dual-ring 1.2s linear infinite;
}
@keyframes lds-dual-ring
{
  0%
  {
    transform: rotate(0deg);
  }
  100%
  {
    transform: rotate(360deg);
  }
}