.shadow {
  position: relative;
  margin: 200px auto 0;
  width: 400px;
  height: 250px;
  background: linear-gradient(0deg,#000,#262626);
}

.shadow:before, .shadow:after {
  content: '';
  position: absolute;
  top: -2px;
  left: -2px;
  background: linear-gradient(45deg,#fb0094,#0000ff,#00ff00,#ffff00,#ff0000,#fb0094,#0000ff,#00ff00,#ffff00,#ff0000);
  background-size: 400%;
  width: calc(100% + 4px);
  height: calc(100% + 4px);
  z-index: -1;
  animation: animate 20s linear infinite;
}

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