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

body {
  margin: 0;
  padding: 0;
  background-color: #000;
}

.block {
  position: relative;
  margin: 300px auto 0;
  width: 500px;
  height: 250px;
  background: linear-gradient(0deg, #000, #272727);
}

.block:before, .block:after {
  content: '';
  position: absolute;
  left: -2px;
  top: -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: steam 20s linear infinite;
}

@keyframes steam {
  0% {
    background-position: 0 0;
  }
  50% {
    background-position: 400% 0;
  }
  100% {
    background-position: 0 0;
  }
}

.block:after {
  filter: blur(50px);
}