<div class="gradient-border" id="box">Animated <br>CSS<br>Gradient Border</div>

@import url('https://fonts.googleapis.com/css?family=Raleway:200');

html, body {
  height: 100%;
}
body {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100%;
  background: #1D1F20;
}
#box {
  display: flex;
  align-items: center;
  justify-content: center;
  width: 400px;
  height: 200px;
  color: white;
  font-family: 'Raleway';
  font-size: 2.5rem;
}
.gradient-border {
  --borderWidth: 3px;
  background: #1D1F20;
  position: relative;
  border-radius: var(--borderWidth);
}
.gradient-border:after {
  content: '';
  position: absolute;
  top: calc(-1 * var(--borderWidth));
  left: calc(-1 * var(--borderWidth));
  height: calc(100% + var(--borderWidth) * 2);
  width: calc(100% + var(--borderWidth) * 2);
  background: linear-gradient(60deg, #f79533, #f37055, #ef4e7b, #a166ab, #5073b8, #1098ad, #07b39b, #6fba82);
  border-radius: calc(2 * var(--borderWidth));
  z-index: -1;
  animation: animatedgradient 3s ease alternate infinite;
  background-size: 300% 300%;
}


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