<div class="shadow">Welcome</div>

:root {
	--gradient-shadow: linear-gradient(
		45deg,
		#fb0094,
		#0000ff,
		#00ff00,
		#ffff00,
		#ff0000,
		#fb0094,
		#0000ff,
		#00ff00,
		#ffff00,
		#ff0000
	);
}
body {
	margin: 0;
	padding: 0;
	background: #000;
	display: flex;
	justify-content: center;
	align-items: center;
	height: 100vh;
}
.shadow {
	display: flex;
	justify-content: center;
	align-items: center;
	position: relative;
	color: #fff;
	text-align: center;
	font-size: 50px;
	font-weight: bold;
	width: 400px;
	height: 250px;
	background: linear-gradient(0deg, #000, #262626);
}
.shadow:before,
.shadow:after {
	content: "";
	position: absolute;
	top: -2px;
	left: -2px;
	background: var(--gradient-shadow);
	background-size: 400%;
	width: calc(100% + 4px);
	height: calc(100% + 4px);
	z-index: -1;
	animation: animate 20s linear infinite;
}

.shadow:after {
	filter: blur(20px);
}

@keyframes animate {
	0% {
		background-position: 0 0;
	}
	50% {
		background-position: 300% 0;
	}
	100% {
		background-position: 0 0;
	}
}

/* No JS Needed */