index.css

html, body {
  display: flex;
  height: 100vh;
  align-content: center;
  width: 100vw;
}
.front {
  background-color: blue;
}

.back {
  background-color: gray;
  transform: rotateY(180deg);
}

.front, .back{
  backface-visibility: hidden;
  color: white;
  font-family: Helvetica;
  font-size: 100px;
  font-weight: 900;
  height: 100%;
  position: absolute;
  width: 100%;
}

.value {
  height: 100%;
  overflow: hidden;
  width: 100%;
}

.card {
  box-shadow: 0 4px 8px 0 rgba(0,0,0,0.4);
  height: 100px;
  position: relative;
  transition: transform 0.8s;
  transform-style: preserve-3d;
  width: 100px;
}

.card.flipped {
  transform: rotateY(180deg);
}

game-board {
  display: flex;
  flex-flow: row wrap;
  justify-content: space-around;
  margin: auto;
  perspective: 500px;
  width: 75vw;
}

game-card {
  margin-bottom: 10px;
}