<div class="block"></block>

body {
	margin: 0;
	padding: 0;
  width: 100vw;
  min-height: 100vh;
	background-color: #000;
  display: flex;
  justify-content: center;
  align-items: center;
}

.block {
	position: relative;
	width: 50vw;
	height: 60vh;
	background: linear-gradient(0deg, #000, #272727);
}

.block:before, 
.block:after {
	content: '';
	position: absolute;
	left: -.5em;
	top: -.5em;
	background: linear-gradient(45deg, #fb0094, #0000ff, #00ff00,#ffff00, #ff0000, #fb0094, 
		#0000ff, #00ff00,#ffff00, #ff0000);
	background-size: 400%;
	width: calc(100% + 1em);
	height: calc(100% + 1em);
	z-index: -1;
	animation: hue 1s linear infinite;
  border-radius: 5px;
}

@keyframes hue {
  from {
    filter: hue-rotate(0deg);
  }
  to {
    filter: hue-rotate(360deg);
  }
}