style.css

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

body {
    height: 100vh;
    background-color: blueviolet;
    display: flex;
    justify-content: center;
    align-items: center;
}

.memory-game {
    border: 3px solid white;
    width: 640px;
    height: 500px;
    display: flex;
    flex-wrap: wrap;
    border-radius: 5px;
    perspective: 1000px;
    
}

.memory-card {
    width: calc(25% - 10px);
    height: calc(33.333% - 10px);
    position: relative;
    margin: 5px;
    cursor: pointer;
    user-select: none;
    transform: scale(1);
    transform-style: preserve-3d;
    transition: transform .4s;
}

.memory-card:active {
    transform: scale(.97);
    transition: transform .2s;
}

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

.front-face, .back-face {
    position: absolute;
    width: 100%;
    height: 100%;
    padding: 20px;
    background-color: white;
    border-radius: 3px;
    backface-visibility: hidden;
}

.front-face {
    transform: rotateY(180deg);
    padding: 10px;
}