div.loader
  - for (var i = 0; i < 22; ++i) {
    div.line
  - }

$animation-length: 4s;
$size: 80vh;

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
html, body {
  display: flex;
  justify-content: center;
  align-items: center;
  width: 100%;
  height: 100%;
  background: rgb(50,50,50);
}
.loader {
  position: relative;
  display: block;
  width: $size;
  height: $size;
}
.line {
  height: $size/45;
  background: #ccc;
  margin-bottom: $size/41;  
  animation: $animation-length load ease-in-out infinite;
  @for $i from 1 through 21 {
    &:nth-of-type(#{$i+1}) {
      animation-delay: -#{$i/10}s;
    }
  }
}

@keyframes load {
  0% {
    width: 25%;
    margin-left: 75%;
  }
  25% {
    width: 1%;
    margin-left: 50%;
  }
  50% {
    width: 25%;
    margin-left: 0;
  }
  75% {
    width: 1%;
    margin-left: 50%;
  }
  100% {
    width: 25%;
    margin-left: 75%;
  }
}