:root {
  --color-dark: #444;
  --color-light: #ecc026;
  --di: min(100vh, 100vw);
  --board-di: calc(var(--di) * 0.66666666);
  --border: calc(var(--board-di) * 0.013333333);
  --dimension-lg: calc(var(--line) * 14); 
  --dimension-sm: calc(var(--line) * 7); 
  --line: var(--border);
}

#alert {
  --color: var(--color-dark);
  background-color: rgba(255, 255, 255, 0.8);
  bottom: 0;
  left: 0;
  opacity: 0;
  position: fixed;
  right: 0;
  top: 0;
  transition: opacity 250ms ease-in-out;
}
#alert:not(.active) {
  pointer-events: none;
}

#alert div {
  background-color: var(--color);
  border-radius: var(--border);
  box-sizing: border-box;
  color: white;
  cursor: pointer;
  font-size: calc(var(--board-di) * 0.07);
  font-weight: bold;
  left: 50%;
  opacity: 0;
  padding: calc(var(--border) * 2);
  position: absolute;
  text-align: center;
  top: 50%;
  transform: scale(0.00001) translate(-50%, -50%);
  transform-origin: 0 0;
  transition: all 250ms ease-in-out;
  width: calc(var(--board-di) * 0.75);
}

#alert.active,
#alert.active div {
  opacity: 1;
}

#alert.active div {
  transform: scale(1) translate(-50%, -50%);
}

section {
  box-sizing: border-box;
  height: var(--di);  
  margin: calc((100vh - var(--di)) * 0.5) calc((100vw - var(--di)) * 0.5);
  padding: calc((var(--di) - var(--board-di)) * 0.5);
  width: var(--di);
}

#board {
  display: flex;
  flex-wrap: wrap;
  height: var(--board-di);
  position: relative;
  width: var(--board-di);
}

#board .tile {
  display: block;
  height: 25%;
  position: relative;
  width: 25%;
}

#board .tile::after {
  align-items: center;
  background: #e9e9e9;
  border-radius: calc(var(--border) * 1.5);
  color: white;
  content: attr(label);
  display: flex;
  flex-direction: column;
  font-size: calc(var(--board-di) * 0.07);
  font-weight: bold;
  justify-content: center;
  height: 90%;
  left: 5%;
  position: absolute;
  top: 5%;
  width: 90%;
}

#board .piece {
  --initial-x: 0;
  --initial-y: -1;
  background-position: center;
  background-color: #fff;
  background-image: repeating-linear-gradient(
    45deg,
    var(--color-piece),
    var(--color-piece) var(--line),
    transparent var(--line),
    transparent calc(var(--line) * 2)
  );
  border: var(--border) solid transparent;
  box-sizing: border-box;
  height: var(--dimension);
  left: calc(var(--x, var(--initial-x)) * 25% + 12.5%);
  position: absolute;
  top: calc(var(--y, var(--initial-y)) * 25% + 12.5%);
  transform: translate(-50%, -50%);
  transition: all 150ms ease-in-out;
  width: var(--dimension);
}

#board .piece.active {
  box-shadow: 0 0 0 var(--border) red;
}

#board .piece.dark { --color-piece: var(--color-dark); }
#board .piece.light { --color-piece: var(--color-light); }
#board .piece.tall { --dimension: var(--dimension-lg); }
#board .piece.short { --dimension: var(--dimension-sm); }
#board .piece.round { border-radius: 50%; }
#board .piece.square { border-radius: var(--border); }
#board .piece.solid { background-color: var(--color-piece); }
#board .piece.hollow { border-color: var(--color-piece); }