@use postcss-preset-env;
@import url(https://fonts.googleapis.com/css?family=Sahitya);

:root {
  --size: 180px;
  --gap: 5px;
  --tdeg: 90deg;
  --dr: 200ms;
  --tf: ease-out;
  --pr: 300px;
  --fr: #000;
  --bg: #fff;
  --mn: #00BCD4;
  --sc: #673AB7;
}

li {
  margin: var(--gap);
  position: relative;
  width: var(--size);
  height: var(--size);
  display: inline-block;
  vertical-align: top;
  perspective: var(--pr);
  transform-origin: 50% 50% calc(var(--size)/2);
}

.w {
  font-size: initial;
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  pointer-events: none;
  transform-style: preserve-3d;
  transform-origin: 50% 50% calc(var(--size) / 2 * -1);
  will-change: transform;
  animation: var(--dr) var(--tf) 0ms 1 normal forwards paused;
}

.f,
.b {
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  color: white;
  transition: none;
}

.f {
  background-color: var(--mn);
  background: linear-gradient(var(--mn), var(--sc));
  transform: translate3d(0,0,0);

  & > svg {
    mix-blend-mode: luminosity;
  }
}

.b {
  padding: 1rem;
  background-image: radial-gradient(circle, #333, #000 160%);
  transform: translate3d(0,0,-1px);

  display: flex;
  align-items: center;
  justify-content: center;
  text-shadow: 0 20px 20px black;
  text-align: center;
}

.in-top .b,
.out-top .b {
  transform-origin: 0% 100%;
  transform: translate3d(0, -100%, 0) rotate3d(1,0,0,90deg);
}

.in-right .b,
.out-right .b {
  transform-origin: 0% 0%;
  transform: translate3d(100%, 0, 0) rotate3d(0,1,0,90deg);
}

.in-bottom .b,
.out-bottom .b {
  transform-origin: 0% 0%;
  transform: translate3d(0, 100%, 0) rotate3d(-1,0,0,90deg);
}

.in-left .b,
.out-left .b {
  transform-origin: 100% 0;
  transform: translate3d(-100%,0,0) rotate3d(0,-1,0,90deg);
}

.in-top .w    {animation-name: in-top; animation-play-state: running}
.in-right .w  {animation-name: in-right; animation-play-state: running}
.in-bottom .w {animation-name: in-bottom; animation-play-state: running}
.in-left .w   {animation-name: in-left; animation-play-state: running}

.out-top .w    {animation-name: out-top; animation-play-state: running}
.out-right .w  {animation-name: out-right; animation-play-state: running}
.out-bottom .w {animation-name: out-bottom; animation-play-state: running}
.out-left .w   {animation-name: out-left; animation-play-state: running}

@keyframes in-top {
  from  {transform: rotate3d(0,0,0,0deg)}
  to    {transform: rotate3d(-1,0,0,var(--tdeg))}
}
@keyframes out-top {
  from {transform: rotate3d(-1,0,0,var(--tdeg))}
  to   {transform: rotate3d(0,0,0,0deg)}
}

@keyframes in-right {
  from  {transform: rotate3d(0,0,0,0deg)}
  to    {transform: rotate3d(0,-1,0,var(--tdeg))}
}
@keyframes out-right {
  from  {transform: rotate3d(0,-1,0,var(--tdeg))}
  to    {transform: rotate3d(0,0,0,0deg)}
}

@keyframes in-bottom {
  from  {transform: rotate3d(0,0,0,0deg)}
  to    {transform: rotate3d(1,0,0,var(--tdeg))}
}
@keyframes out-bottom {
  from  {transform: rotate3d(1,0,0,var(--tdeg))}
  to    {transform: rotate3d(0,0,0,0deg)}
}

@keyframes in-left {
  from  {transform: rotate3d(0,0,0,0deg)}
  to    {transform: rotate3d(0,1,0,var(--tdeg))}
}
@keyframes out-left {
  from  {transform: rotate3d(0,1,0,var(--tdeg))}
  to    {transform: rotate3d(0,0,0,0deg)}
}

/* ======= aesthetics ======= */

* {
  margin: 0;
  box-sizing: border-box;
}

body {
  padding: 3rem 1.5rem;
  font-size: 16px;
  font-family: 'Sahitya', serif;
  color: var(--fr);
  background-color: var(--bg);
  line-height: 1.618;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

header {
  padding-bottom: 3rem;
}

h1 {
  font-size: 2em;

  & sup {
    font-size: .7em;
    color: var(--mn);
  }
}

ul {
  font-size: 0;
  padding: 0;
  max-width: calc((var(--size) + (var(--gap) * 2)) * 4);
  margin: 0 auto;
  list-style: none;

  &::after {
    content: '';
    display: table;
    clear: both;
  }
}

footer {
  font-size: .75rem;
  font-family: monospace;
  padding-top: 3rem;

  & a {
    display: inline-block;
    vertical-align: middle;
  }

  & svg {fill: currentColor}
}

:any-link {
  color: var(--mn);
}

.-center {
  text-align: center;
}

.-neutral-mid {
  color: color(var(--fr) tint(60%));
}