style.css

:root {
  --R: 50vh;
  --I: 350px;
}

.body {
  margin: 0;
  overflow: hidden;
  background-color: #000;
  -webkit-tap-highlight-color: transparent;
}

#stage {
  display: block;
  filter: grayscale(100%);
  transition: filter 0.666s;
}

#shadow {
  position: absolute;
  width: calc(var(--R) * 10);
  height: calc(var(--R) * 10);
  margin-top: calc(var(--R) * -5);
  margin-left: calc(var(--R) * -5);
  top: 50vh;
  left: 50%;
  background: radial-gradient(#333, #222 30%, #111 60%, #000000 68%);
}

#items {
  position: absolute;
  width: 100%;
  height: 100%;
  list-style: none;
  margin: 0;
  padding: 0;
  transform: translate(calc(45% - var(--I) / 2), calc(50vh - var(--I) / 2));
}

.item {
  position: absolute;
  height: calc(var(--I));
  width: calc(var(--I));
  z-index: 10;
}

.item:nth-child(1) {
  clip-path: polygon(40% 10%, 100% 0, 90% 75%, 0 100%);
}

.item:nth-child(2) {
  left: calc(var(--I));
  clip-path: polygon(5% 0, 90% 25%, 100% 100%, 0 40%);
}

.item:nth-child(3) {
  left: calc(var(--I) * 0.8);
  top: calc(var(--I) * 0.45);
  clip-path: polygon(17.5% 0, 100% 50%, 10% 70%);
}

.item:nth-child(4) {
  top: calc(var(--I) * 0.8);
  left: calc(var(--I) * -0.1);
  clip-path: polygon(20% 25%, 100% 0, 80% 100%, 25% 65%);
}

.item:nth-child(5) {
  top: calc(var(--I) * 0.8);
  left: calc(var(--I) * -0.95);
  clip-path: polygon(70% 0, 100% 45%, 95% 85%, 25% 80%);
}

.item:nth-child(6) {
  top: calc(var(--I) * -0.1);
  left: calc(var(--I) * -0.75);
  clip-path: polygon(40% 0, 100% 45%, 70% 100%, 40% 60%);
}

.item:nth-child(7) {
  left: calc(var(--I) * -0.3);
  top: calc(var(--I) * -0.7);
  clip-path: polygon(75% 30%, 55% 100%, 0  60%);
}

.item:nth-child(8) {
  left: calc(var(--I) * 0.4);
  top: calc(var(--I) * -0.95);
  clip-path: polygon(25% 0, 100% 100%, 65% 90%, 0% 100%);
}

.item:nth-child(9) {
  top: calc(var(--I) * 0.5);
  left: calc(var(--I) * -1.25);
  clip-path: polygon(85% 0, 100% 20%, 75% 60%, 0 40%);
}

.item:nth-child(10) {
  top: calc(var(--I) * -0.69);
  left: calc(var(--I) * 1.2);
  clip-path: polygon(75% 0, 100% 100%, 25% 75%, 0 40%);
}

.image {
  height: 105%;
  width: 105%;
  background-size: auto 100%;
}

#guide {
  font-family: 'DM Sans', 'Zen Kaku Gothic New', sans-serif;
  color: #ffffffbb;
  text-shadow: 0 0 10px #000;
  font-size: calc(var(--R) * 0.15);

  opacity: 0;
  transition: opacity 0.5s ease-in-out;
  z-index: 10;
  pointer-events: none;

  position: absolute;
  display: flex;
  align-items: center;
  justify-content: center;
  width: 100%;
  height: 100%;
}

[hidden] {
  display: none;
}

/* PCサイズ */
@media screen and (min-width: 768px) {
  :root {
    --I: 400px;
  }
}