memorymatch.css

:root {
  box-sizing: border-box;
}

*,
*::before,
*::after {
  box-sizing: inherit;
}

body {
  margin: 0;
  padding: 0;
  height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
}

.board {
  width: 900px;
  display: flex;
  flex-wrap: wrap;
}

.board > div {
  background-color: lightslategray;
  margin: 10px;
  width: 200px;
  height: 200px;
  box-shadow:
    3px 6px 24px #ccc,
    5px 5px 13px #ccc;
}

/* Animation by Sarah Drasner */
/* https://css-tricks.com/snippets/css/shake-css-keyframe-animation */
.shake {
  animation:
    shake 0.82s cubic-bezier(.36,.07,.19,.97) both;
  perspective: 1000px;
  transform: translate3d(0, 0, 0);
  backface-visibility: hidden;
}

@keyframes shake {
  10%, 90% {
    transform: translate3d(-1px, 0, 0);
  }
  
  20%, 80% {
    transform: translate3d(2px, 0, 0);
  }

  30%, 50%, 70% {
    transform: translate3d(-4px, 0, 0);
  }

  40%, 60% {
    transform: translate3d(4px, 0, 0);
  }
}