style.css

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

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

/* [#] -> selects the div's id */
/* [.] -> selects the div's class */

.block {
    position: relative;
    width: 300px;
    height: 300px;
    transform-style: preserve-3d;
    transform: rotateX(-30deg);
    animation: animate 4s linear infinite;
}

@keyframes animate
{
    0%
    {
        transform: rotateX(-30deg) rotateY(0deg);
    }
    100%
    {
        transform: rotateX(-30deg) rotateY(360deg);
    }
}

.item {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    transform-style: preserve-3d;
}

.item span {
    position:absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: linear-gradient(black,cyan);
    transform: rotateY(calc(90deg * var(--k))) translateZ(150px); 
    /* 300px divided into 2 */
}

.top {
    position: absolute;
    top: 0;
    left: 0;
    width: 300px;
    height: 300px;
    background: rgb(48, 47, 47);
    transform: rotateX(90deg) translateZ(150px);
}

/* ::before -> creates a pseudo element(first child of selected element [top])  */
/* .top::before {} */

.base {
    content: "";
    position: absolute;
    top: 0;
    left: 0;
    width: 300px;
    height: 300px;
    background: cyan;
    transform: rotateX(90deg) translateZ(-220px);
    filter: blur(15px);
    box-shadow: 0 0 120px rgb(0, 32, 32),
    0 0 200px rgb(0, 53, 53),
    0 0 300px rgb(0, 99, 99),
    0 0 400px rgb(0, 173, 173),
    0 0 500px cyan;
}