SOURCE

<div id="out"></div>

#out {
    background: #fc0;
    border-radius: 20px;
    position: relative;
	width: 100px;
	height: 100px;
	flex-shrink: 0;
	margin: 50px;
    overflow: unset;
}

#out::before,
#out::after {
    content: '';
    position: absolute;
    left: -6px;
    border-radius: 20px;
    top: -6px;
    width: calc(100% + 12px);
    height: calc(100% + 12px);
    background: #333;
    background-image: linear-gradient(45deg, #ff3c41, #ff8a25, #ffe04f, #4fff88, #56ffef, #37b4ff, #c05aff, #ff3c41, #ff8a25, #ffe04f, #4fff88, #56ffef, #37b4ff, #c05aff);
    background-size: 500%;
    animation: position 12s linear infinite alternate;
    z-index: -1;
}

#out::after {
    filter: blur(10px);
}

@keyframes position {
    0% {
        background-position: 0 0;
    }
    50% {
        background-position: 400% 0;
    }
    100% {
        background-position: 0 0;
    }
}