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

.book {
    --sw: 0.5em; /* stroke width */
    width: 15em;
    height: 10em;
    background-color: lightyellow;
    box-sizing: border-box;
    border: var(--sw) solid cadetblue;
    border-radius: var(--sw);
    font-size: 20px;
    position: relative;
}

.book .page {
    height: inherit;
    width: calc(50% + var(--sw) + var(--sw) / 2);
    background-color: inherit;
    box-sizing: border-box;
    border: inherit;
    border-radius: inherit;
    position: absolute;
    top: calc(-1 * var(--sw));
    right: calc(-1 * var(--sw));
    display: flex;
    flex-direction: column;
    justify-content: space-between;
    padding: 8% 5%;
    transform-origin: left;
    transform-style: preserve-3d;
    animation: flip 4s linear infinite;
}

.book .page span {
    display: block;
    width: 100%;
    border-top: var(--sw) solid cadetblue;
    border-radius: inherit;
    transform-origin: left;
    transform: scaleX(0);
    backface-visibility: hidden;
    animation: 4s linear infinite;
}

.book .page span:nth-child(1) {animation-name: stroke-1;}
.book .page span:nth-child(2) {animation-name: stroke-2;}
.book .page span:nth-child(3) {animation-name: stroke-3;}
.book .page span:nth-child(4) {animation-name: stroke-4;}
.book .page span:nth-child(5) {animation-name: stroke-5;}

@keyframes stroke-1 {
    0% {transform: scaleX(0);}
    10%, 100% {transform: scaleX(1);}
}

@keyframes stroke-2 {
    0%, 10% {transform: scaleX(0);}
    20%, 100% {transform: scaleX(1);}
}

@keyframes stroke-3 {
    0%, 20% {transform: scaleX(0);}
    30%, 100% {transform: scaleX(1);}
}

@keyframes stroke-4 {
    0%, 30% {transform: scaleX(0);}
    40%, 100% {transform: scaleX(1);}
}

@keyframes stroke-5 {
    0%, 40% {transform: scaleX(0);}
    50%, 100% {transform: scaleX(1);}
}

@keyframes flip {
    55% {
        transform: rotateY(0) translateX(0) skewY(0);
    }

    70% {
        transform: rotateY(-90deg) translateX(calc(-1 * var(--sw) / 2)) skewY(-20deg);
    }

    80%, 100% {
        transform: rotateY(-180deg) translateX(calc(-1 * var(--sw))) skewY(0);
    }
}