<div class="preloader">
  <div class="box">
    <div class="box__inner">
      <div class="box__back-flap"></div>
      <div class="box__right-flap"></div>
      <div class="box__front-flap"></div>
      <div class="box__left-flap"></div>
      <div class="box__front"></div>
    </div>
  </div>
  <div class="box">
    <div class="box__inner">
      <div class="box__back-flap"></div>
      <div class="box__right-flap"></div>
      <div class="box__front-flap"></div>
      <div class="box__left-flap"></div>
      <div class="box__front"></div>
    </div>
  </div>
  <div class="line">
    <div class="line__inner"></div>
  </div>
  <div class="line">
    <div class="line__inner"></div>
  </div>
  <div class="line">
    <div class="line__inner"></div>
  </div>
  <div class="line">
    <div class="line__inner"></div>
  </div>
  <div class="line">
    <div class="line__inner"></div>
  </div>
  <div class="line">
    <div class="line__inner"></div>
  </div>
  <div class="line">
    <div class="line__inner"></div>
  </div>
</div>

* {
  border: 0;
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}
:root {
  --dur: 2s;
  --bg: #e3e4e8;
  --fg: #17181c;
  font-size: calc(12px + (24 - 12)*(100vw - 320px)/(2560 - 320));
}
body {
  background: var(--bg);
  color: var(--fg);
  display: flex;
  font: 1em/1.5 sans-serif;
  height: 100vh;
  overflow: hidden;
}
.preloader, .box, .box__inner, .box__inner div  {
  transform-style: preserve-3d;
}
.preloader {
  margin: auto;
  position: relative;
  width: 12em;
  height: 24em;
}
.box, .box__inner, .box__inner div, .line {
  position: absolute;
}
.box, .box__inner div, .line__inner  {
  animation-timing-function: ease-in-out;
  animation-iteration-count: infinite;
}
.box, .line__inner {
  animation-duration: var(--dur);
}
.box__inner div, .line__inner {
  background: var(--fg);
}
/* Boxes */
.box, .box__inner {
  width: 100%;
  height: 50%;
}
.box {
  animation-name: popOut;
  top: 50%;
  perspective: 25em;
  transform-origin: 50% 75%;
  z-index: 1;
}
.box + .box {
  animation-delay: calc(var(--dur)/-2);
}
.box__inner, .box__inner div  {
  animation-delay: inherit;
  animation-duration: inherit;
}
.box__inner {
  transform: rotateX(50deg);
  transform-origin: 50% 25%;
}
.box__inner div {
  box-shadow: 0 0 0 0.2em var(--bg) inset;
}
.box__back-flap, .box__front-flap, .box__front {
  width: 6em;
}
.box__left-flap, .box__right-flap, .box__front {
  height: 6em;
}
.box__back-flap, .box__front-flap {
  left: 3em;
  height: 3em;
}
.box__left-flap, .box__right-flap {
  top: 3em;
  width: 3em;
}
.box__back-flap {
  animation-name: backFlap;
  transform: rotateX(-180deg);
  transform-origin: 50% 100%;
}
.box__right-flap {
  animation-name: rightFlap;
  left: 9em;
  transform: rotateY(-179deg);
  transform-origin: 0 50%;
}
.box__front-flap {
  animation-name: frontFlap;
  top: 9em;
  transform: rotateX(180deg);
  transform-origin: 50% 0;
}
.box__left-flap {
  animation-name: leftFlap;
  transform: rotateY(179deg);
  transform-origin: 100% 50%;
}
.box__front {
  top: 3em;
  left: 3em;
  transform: rotateX(-90deg) translateY(50%) translateZ(3em);
}
/* Lines */
.line, .line__inner {
  border-radius: 0.1em;
}
.line {
  top: 0;
  left: calc(50% - 0.1em);
  width: 0.2em;
  height: 6em;
  overflow: hidden;
  transform-origin: 50% 0.1em;
}
.line:nth-child(n + 5) {
  top: 90%;
  transform: translateY(2em);
}
.line:nth-child(3) {
  left: calc(50% - 1.1em);
}
.line:nth-child(4) {
  left: calc(50% + 0.9em);
}
.line:nth-child(5) {
  transform: rotate(-90deg) translateY(2em);
}
.line:nth-child(6) {
  transform: rotate(-45deg);
}
.line:nth-child(7) {
  transform: translateY(-0.5em);
}
.line:nth-child(8) {
  transform: rotate(45deg);
}
.line:nth-child(9) {
  transform: rotate(90deg) translateY(2em);
}
.line__inner {
  animation-duration: calc(var(--dur)/2);
  width: 100%;
  height: 100%;
}
.line:nth-child(n + 3):nth-child(-n + 4) .line__inner {
  animation-name: falling;
}
.line:nth-child(n + 5):nth-child(-n + 9) .line__inner {
  animation-name: landing;
  transform: translateY(-100%);
}
/* Dark Mode */
@media (prefers-color-scheme: dark) {
  :root {
    --bg: #17181c;
    --fg: #e3e4e8;
  }
}
/* Animations */
@keyframes backFlap {
  from, 33% { transform: rotateX(-180deg); }
  41% { transform: rotateX(10deg); }
  45% { transform: rotateX(-27deg); }
  50%, to { transform: rotateX(-15deg); }
}
@keyframes rightFlap {
  from, 33% { transform: rotateY(-179deg); }
  41% { transform: rotateY(10deg); }
  45% { transform: rotateY(-27deg); }
  50%, to { transform: rotateY(-15deg); }
}
@keyframes frontFlap {
  from, 33% { transform: rotateX(180deg); }
  41% { transform: rotateX(-10deg); }
  45% { transform: rotateX(27deg); }
  50%, to { transform: rotateX(15deg); }
}
@keyframes leftFlap {
  from, 33% { transform: rotateY(179deg); }
  41% { transform: rotateY(-10deg); }
  45% { transform: rotateY(27deg); }
  50%, to { transform: rotateY(15deg); }
}
@keyframes popOut {
  from, 21% { transform: translateZ(1px) translateY(0) scale(0,0); }
  33% { transform: translateZ(1px) translateY(-100%) scale(0.75,1.5); }
  41% { transform: translateZ(1px) translateY(-100%) scale(1,1); }
  42%,50% { transform: translateZ(0) translateY(-100%) scale(1,1); }
  55% { transform: translateZ(0) translateY(-100%) scale(0.75,1.5); }
  60% { transform: translateZ(0) translateY(0) scale(1.5,0.75); }
  67% { transform: translateZ(0) translateY(0) scale(1,1); }
  to { transform: translateZ(0) translateY(0) scale(0,0); }
}
@keyframes falling {
  from, 12% { transform: translateY(0%); }
  28%, to { transform: translateY(100%); }
}
@keyframes landing {
  from, 17% { transform: translateY(-100%); }
  41%, to { transform: translateY(100%); }
}