style.css

:root {
  --border-width: calc(var(--diameter-tile) / 60);
  --diameter-board: min(85vw, 85vh);
  --diameter-tile: calc(1 / 8 * var(--diameter-board));
  --edge-width: calc((min(100vw, 100vh) - var(--diameter-board)) * 0.3);
  --color-danger: tomato;
  --color-success: #1d83e0;
  --color-white: #f0f0f0;
  --color-black: #222;
  --color-board-hue: 30;
  --color-board-sat: 40%;
  --color-shadow: hsl(var(--color-board-hue), var(--color-board-sat), 50%);
  --color-shadow-lighter: hsl(var(--color-board-hue), var(--color-board-sat), 55%);
  --transition-ease: cubic-bezier(0.25, 1, 0.5, 1);
  --color-background: var(--color-black);
}

aside {
  display: flex;
  justify-content: space-between;
  left: 0;
  position: absolute;
  top: calc(var(--edge-width) * -0.55);
  transform: translateY(-50%);
  width: 100%;
  z-index: 999;
}
aside div {
  align-items: center;
  color: white;
  display: flex;
}
aside div > * {
  align-items: center;
  display: flex;
}
aside div > * + * {
  margin-left: calc(var(--border-width) * 2);
}
aside div h3,
aside div label {
  font-size: calc(var(--edge-width) * 0.3);
  height: calc(var(--edge-width) * 0.3);
  line-height: 1;
  margin-bottom: 0;
  margin-top: 0;
  text-transform: uppercase;
}
aside div label {
  cursor: pointer;
}
aside div input {
  left: -99999px;
  position: absolute;
}

aside div input + * {
  opacity: 0.5;
}
aside div input:checked + * {
  font-weight: bold;
  opacity: 1;
}

aside div svg {
  height: calc(var(--edge-width) * 0.5);
  width: auto;
}

html,
body {
  height: 100%;
}

body {
  background: var(--color-background);
  overflow: hidden;
  transition: background-color 270ms ease-in-out;
}

#view {
  background: var(--color-shadow-lighter);
  box-shadow: 0 0 0 calc(var(--border-width) * 3) var(--color-shadow-lighter),
    0 0 0 var(--edge-width) var(--color-shadow);
  height: var(--diameter-board);
  margin: calc((100vh - var(--diameter-board)) * 0.5)
    calc((100vw - var(--diameter-board)) * 0.5);
  position: relative;
  width: var(--diameter-board);
}

.board {
  display: flex;
  flex-direction: column-reverse;
  height: 100%;
  width: 100%;
}

.board .row {
  display: flex;
  height: var(--diameter-tile);
  width: 100%;
}

.perspective-black .board .row {
  flex-direction: row-reverse;
}

.perspective-black .board {
  flex-direction: column;
}

.board .row .tile {
  background-color: currentcolor;
  border: none;
  box-shadow: inset 0 0 0 var(--border-width) var(--color-shadow-lighter);
  display: flex;
  flex-direction: column;
  height: var(--diameter-tile);
  justify-content: space-between;
  padding: 0;
  position: relative;
  transition: background-color 350ms var(--transition-ease);
  width: var(--diameter-tile);
}

.perspective-black .board .row:nth-child(even) .tile:nth-child(odd),
.perspective-black .board .row:nth-child(odd) .tile:nth-child(even),
.perspective-white .board .row:nth-child(even) .tile:nth-child(even),
.perspective-white .board .row:nth-child(odd) .tile:nth-child(odd) {
  color: hsl(var(--color-board-hue), var(--color-board-sat), 62%);
}

.perspective-black .board .row:nth-child(even) .tile:nth-child(even),
.perspective-black .board .row:nth-child(odd) .tile:nth-child(odd),
.perspective-white .board .row:nth-child(even) .tile:nth-child(odd),
.perspective-white .board .row:nth-child(odd) .tile:nth-child(even) {
  color: hsl(var(--color-board-hue), var(--color-board-sat), 70%);
}

.board .row .tile .move,
.board .row .tile .moves,
.board .row .tile .captures {
  box-sizing: border-box;
  display: flex;
  flex-wrap: wrap;
  height: var(--diameter-tile);
  justify-content: center;
  left: 0;
  padding: calc(var(--diameter-tile) * 0.025);
  position: absolute;
  top: 0;
  width: var(--diameter-tile);
  z-index: 9;
}

.board .row .tile .move,
.board .row .tile .moves {
  align-content: center;
  align-items: center;
}
.board .row .tile .captures {
  align-items: flex-start;
  justify-content: space-between;
}
.board .row .tile:not(.occupied) .captures {
  align-items: center;
  justify-content: center;
}

.board .row .tile > div > svg {
  --stroke: transparent;
  box-sizing: border-box;
  height: var(--di);
  line-height: var(--di);
  width: var(--di);
}

.board .row .tile .move svg {
  --di: calc(var(--diameter-tile) / 4);
  --fill: var(--color-shadow);
}

.board .row .tile .moves svg,
.board .row .tile .captures svg {
  --di: calc(var(--diameter-tile) / 4);
  --fill: var(--color-shadow);
  opacity: 0.4;
}

.board .row .tile.occupied .captures svg { position: absolute; }
.board .row .tile.occupied .captures svg:nth-child(1) { top: 0; left: 0; }
.board .row .tile.occupied .captures svg:nth-child(2) { top: 0; right: 0; }
.board .row .tile.occupied .captures svg:nth-child(3) { bottom: calc(var(--di) * 0.1); left: 0; }
.board .row .tile.occupied .captures svg:nth-child(4) { bottom: calc(var(--di) * 0.1); right: 0; }
.board .row .tile.occupied .captures svg:nth-child(5) { top: calc(50% - var(--di) * 0.55); left: 0; }
.board .row .tile.occupied .captures svg:nth-child(6) { top: calc(50% - var(--di) * 0.55); right: 0; }
.board .row .tile.occupied .captures svg:nth-child(7) { top: 0; left: calc(50% - var(--di) * 0.5); }
.board .row .tile.occupied .captures svg:nth-child(8) { bottom: calc(var(--di) * 0.1); left: calc(50% - var(--di) * 0.5); }

.touching .board .row .tile .moves,
.touching .board .row .tile .captures,
.turn-black .board .row .tile .moves .white,
.turn-black .board .row .tile .captures .white,
.turn-white .board .row .tile .moves .black,
.turn-white .board .row .tile .captures .black {
  display: none;
}

.board .row .tile[class*="highlight-"] .moves,
.board .row .tile[class*="highlight-"] .captures {
  display: none;  
}

button:focus {
  outline: none;
  position: relative;
  z-index: 9;
}

svg {
  --fill: var(--color-black);
  --stroke: var(--color-shadow);
  fill: var(--fill);
}

svg.white { --fill: var(--color-white); }
svg.black { --fill: var(--color-black); }

.pieces {
  display: block;
  height: var(--diameter-board);
  left: 0;
  pointer-events: none;
  position: absolute;
  top: 0;
  width: var(--diameter-board);
  z-index: 99;
}

.pieces .piece.white {
  --pos-row: -1;
}
.pieces .piece.black {
  --pos-row: 8;
}
.pieces .piece {
  --pos-col: 3.5;
  --scale: 0;
  --transition-delay: 0ms;
  --transition-duration: 200ms;
  bottom: 0;
  display: block;
  height: var(--diameter-tile);
  position: absolute;
  left: 0;
  transform: translate(
      calc(var(--pos-col) * 100%),
      calc(var(--pos-row) * -100%)
    )
    translateZ(0);
  transform-origin: 50% 50%;
  transition: all var(--transition-duration) var(--transition-ease)
    var(--transition-delay);
  width: var(--diameter-tile);
}
.perspective-black .pieces .piece {
  transform: translate(
      calc((7 - var(--pos-col)) * 100%),
      calc((7 - var(--pos-row)) * -100%)
    )
    translateZ(0);
}
.pieces .piece svg {
  display: block;
  left: 50%;
  opacity: 1;
  position: absolute;
  top: 50%;
  transform: translate(-50%, -50%) translateZ(0) scale(var(--scale));
  transform-origin: 50% 50%;
  transition: transform var(--transition-duration) var(--transition-ease),
    fill var(--transition-duration) var(--transition-ease),
    opacity var(--transition-duration) var(--transition-ease);
}
.turn-white .pieces .piece:not(.highlight-capture) svg.black,
.turn-black .pieces .piece:not(.highlight-capture) svg.white,
.turn-black .pieces .piece:not(.can-move):not(.can-capture) svg.black,
.turn-white .pieces .piece:not(.can-move):not(.can-capture) svg.white {
  --stroke: transparent;
  opacity: 0.8;
}

@-webkit-keyframes wobble {
  0%, 50%, 100% { transform: translate(-50%, -50%) translateZ(0) scale(1) rotate(0deg); }
  25% { transform: translate(-50%, -50%) translateZ(0) scale(1.1) rotate(-2deg); }
  75% { transform: translate(-50%, -50%) translateZ(0) scale(1.1) rotate(2deg); }
}

@keyframes wobble {
  0%, 50%, 100% { transform: translate(-50%, -50%) translateZ(0) scale(1) rotate(0deg); }
  25% { transform: translate(-50%, -50%) translateZ(0) scale(1.1) rotate(-2deg); }
  75% { transform: translate(-50%, -50%) translateZ(0) scale(1.1) rotate(2deg); }
}
.pieces .piece.highlight-active svg {
  -webkit-animation: wobble 500ms linear infinite;
          animation: wobble 500ms linear infinite;
  --stroke: var(--color-success);
}

.pieces .piece.highlight-capture svg {
  --stroke: var(--color-danger);
}

.piece svg {
  --svg-di: calc(var(--diameter-tile) * 0.666);
  display: block;
  font-weight: bold;
  height: var(--svg-di);
  left: 50%;
  line-height: var(--svg-di);
  position: absolute;
  stroke-linejoin: round;
  text-align: center;
  top: 50%;
  transform: translate(-50%, -50%);
  width: var(--svg-di);
}