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

.flag {
    width: 10em;
    height: 15em;
    font-size: 20px;
    display: flex;
    justify-content: space-between;
    transform: perspective(500px) rotateY(-20deg);
}

.flag span {
    width: 0.25em;
    height: inherit;
    background-color: ghostwhite;
    animation: wave 1.5s ease-in-out infinite alternate;
    animation-delay: calc(var(--n) * -0.1s);
}

@keyframes wave {
    to {
        transform: translateX(2em);
        background-color: deepskyblue;
    }
}

.flag span:nth-child(1) {
    --n: 1;
}

.flag span:nth-child(2) {
    --n: 2;
}

.flag span:nth-child(3) {
    --n: 3;
}

.flag span:nth-child(4) {
    --n: 4;
}

.flag span:nth-child(5) {
    --n: 5;
}

.flag span:nth-child(6) {
    --n: 6;
}

.flag span:nth-child(7) {
    --n: 7;
}

.flag span:nth-child(8) {
    --n: 8;
}

.flag span:nth-child(9) {
    --n: 9;
}

.flag span:nth-child(10) {
    --n: 10;
}

.flag span:nth-child(11) {
    --n: 11;
}

.flag span:nth-child(12) {
    --n: 12;
}

.flag span:nth-child(13) {
    --n: 13;
}

.flag span:nth-child(14) {
    --n: 14;
}

.flag span:nth-child(15) {
    --n: 15;
}