index.css

body {
    margin: 0;
    color: #434344;
    background-color: #cfd0cf;

    font-family: Arial, sans-serif;
    font-weight: bolder;
}

header {
    margin: 40px 0;
    text-align: center;
}

h1 {
    font-size: 42px;
    line-height: 47px;
}

div.cells {
    perspective: 600px;
}

div.row {
    margin: 25px;

    text-align: center;
    white-space: nowrap;
}

div.row div.cell {
    display: inline-block;
    transform-style: preserve-3d;

    width: 130px;
    height: 130px;
    border-radius: 9px;
    margin-right: 25px;

    transition: transform linear 0.2s;
}

div.row div.cell div {
    position: absolute;
    -webkit-backface-visibility: hidden;
    backface-visibility: hidden;

    width: 100%;
    height: 100%;
    cursor: pointer;

    border-radius: inherit;
    box-shadow: 1px 1px 5px rgba(0, 0, 0, 0.5);
}

div.row div.cell div.front {
    box-sizing: border-box;
    border: 5px solid white;
    background: linear-gradient(to top right, #22ab93, #19668d);
}

div.row div.cell div.back {
    display: flex;
    justify-content: center;
    align-items: center;

    transform: rotateY(0.5turn);

    background-color: white;
    font-size: 75px;
}

div.row div.cell.flipped {
    transform: rotateY(0.5turn);
}

div.row div.cell.flipped.success div.back {
    background-color: #5ad66f;
}

div.row div.cell.flipped.fail div.back {
    background-color: #f44336;
}

div.timer {
    margin-top: 30px;
    text-align: center;

    font-size: 32px;
    line-height: 36px;
}

dialog {
    min-width: 350px;
    padding: 30px 0;
    border: none;

    text-align: center;
}

dialog::backdrop {
    background-color: rgba(0, 0, 0, 0.5);
}

dialog div.result {
    font-size: 48px;
    color: #434344;
}

dialog button {
    height: 40px;

    padding: 0 30px;
    border: none;
    margin-top: 40px;
    box-shadow: 1px 1px 1px rgba(0, 0, 0, 0.5);
    outline: none;

    font-size: 20px;
    color: white;
    background: linear-gradient(to right, #19668d, #22ab93);
}

dialog button:active {
    box-shadow: inset 2px 1px 8px rgba(0, 0, 0, 0.5);
}

/* Animation */
dialog div.result span {
    display: inline-block;

    animation: 1s jump ease-out infinite;
    transform-origin: bottom;
}

dialog div.result span:nth-of-type(2) {
    animation-delay: 0.1s;
}

dialog div.result span:nth-of-type(3) {
    animation-delay: 0.2s;
}

dialog div.result span:nth-of-type(4) {
    animation-delay: 0.3s;
}

@keyframes jump {
    40%,
    50% {
        transform: scaleY(1.4);
        animation-timing-function: ease-in-out;
    }

    90% {
        transform: scaleY(0.7);
        animation-timing-function: linear;
    }
}