HTML :

<div class="myGrad"></div>

---

CSS :

/* 
Attention, si vous augmentez la hauteur de la div myGrad, alors il faudra aussi augmenter la hauteur de myGrad:after
*/

body {
  font-size: 2em;
  color: #FFF;
  text-align: center;
}

html{
  background-color: #34495e;
}
/* main */
/* mixin */
.myGrad {
  width: 100%;
  height: 50vh;
  margin: 20px 0;
  border-top: solid 10px #FFF;
  border-bottom: solid 10px #FFF;
  overflow: hidden;
}
.myGrad:after {
  position: relative;
  content: "";
  display: block;
  top: 0;
  left: 0;
  width: 100%;
  height: 250vh;
  background: linear-gradient(0% -15%, #655bef 0%, #fa28bf 20%, #ff4665 40%, #fbde4e 60%, #00fbea 80%, #555def 100%);
  background: -moz-linear-gradient(0% -15%, #655bef 0%, #fa28bf 20%, #ff4665 40%, #fbde4e 60%, #00fbea 80%, #555def 100%);
  background: -webkit-gradient(linear, 0% -15%, 100% 100%, color-stop(0, #655bef), color-stop(0.2, #fa28bf), color-stop(0.4, #ff4665), color-stop(0.6, #fbde4e), color-stop(0.8, #00fbea), color-stop(1, #555def));
  -o-animation: anime 17.5s infinite;
  -ms-animation: anime 17.5s infinite;
  -moz-animation: anime 17.5s infinite;
  -webkit-animation: anime 17.5s infinite;
}

@-webkit-keyframes anime {
  0% {
    top: 0;
  }
  50% {
    top: -1800px;
  }
  100% {
    top: 0;
  }
}
@keyframes anime {
  0% {
    top: 0;
  }
  50% {
    top: -1800px;
  }
  100% {
    top: 0;
  }
}