<a href="#" class="rainbow-button" alt="Button"></a>

body {
  margin:0;
  background-color:#191919;
  min-height:100vh;
  display:flex;
  align-items:center;
  justify-content:center;
  font-family:Helvetica,Sans-serif;
}
a {
  text-decoration:none;
  color:#FFF;
}
.rainbow-button {
  width:calc(20vw + 6px);
  height:calc(8vw + 6px);
  background-image: linear-gradient(90deg, #00C0FF 0%, #FFCF00 49%, #FC4F4F 80%, #00C0FF 100%);
  border-radius:5px;
  display:flex;
  align-items:center;
  justify-content:center;
  text-transform:uppercase;
  font-size:3vw;
  font-weight:bold;
}
.rainbow-button:after {
  content:attr(alt);
  width:20vw;
  height:8vw;
  background-color:#191919;
  display:flex;
  align-items:center;
  justify-content:center;
}
.rainbow-button:hover {
  animation:slidebg 2s linear infinite;
}

@keyframes slidebg {
  to {
    background-position:20vw;
  }
}