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

body {
  height: 100vh;
  display: flex;
  background-color: #bcc7f8;
}

.memory-game {
  height: 640px;
  width: 840px;
  margin: auto;
  display: flex;
  flex-wrap: wrap;
  perspective: 1000px; /* perspective axis z*/
}

.card {
  height: calc(33.333% - 10px);
  width: calc(25% - 10px);
  margin: 5px;
  position: relative;
  box-shadow: 1px 1px 1px rgba(0, 0, 0, 0.3);
  cursor: pointer;
  transform: scale(1);
  transform-style: preserve-3d; /* apply 3d perspective */
  transition: transform 0.9s; /* flip movement effect */
}

.card:active {
  transform: scale(0.97);
  transition: transform 0.2s;
}

.card-front,
.card-back {
  width: 100%;
  height: 100%;
  padding: 5px;
  position: absolute;
  border-radius: 5px;
  background: #5c7dbb;
  backface-visibility: hidden; /* front-back | no back */
}

.card-front {
  transform: rotateY(180deg);
  /* before, the verse was transparent because of both images with absolute position, 
    after it turns in axis Y, they turn their faces */
}

/* flip card animation */

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