<div class="box">
  Animated Gradient Underglow
</div>

body {
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
  color: #999;
}

.box {
  background: hsl(0, 0%, 100%);
  padding: 16px 24px;
  position: relative;
  border-radius: 8px;
  box-shadow: 0 0 0 1px rgba(0,0,0,.01);

  &::after {
    position: absolute;
    content: "";
    top: 15px;
    left: 0;
    right: 0;
    z-index: -1;
    height: 100%;
    width: 100%;
    transform: scale(0.9) translateZ(0);
    filter: blur(15px);
    background: linear-gradient(
      to left,
      #ff5770,
      #e4428d,
      #c42da8,
      #9e16c3,
      #6501de,
      #9e16c3,
      #c42da8,
      #e4428d,
      #ff5770
    );
    background-size: 200% 200%;
    animation: animateGlow 1.25s linear infinite;
  }
}

@keyframes animateGlow {
  0% {
    background-position: 0% 50%;
  }
  100% {
    background-position: 200% 50%;
  }
}