/* HTML */
<a href="#"><button class="button-03">Join now</button></a>

/* CSS */
.button-03 {
  --width: 150px;
  --timing: 2s;
  border: 0;
  width: var(--width);
  padding-block: 1em;
  color: #fff;
  font-weight: bold;
  font-size: 1em;
  background: rgb(64, 192, 87);
  transition: all 0.2s;
  border-radius: 3px;
}

.button-03:hover {
  background-image: linear-gradient(to right, rgb(250, 82, 82), rgb(250, 82, 82) 16.65%, rgb(190, 75, 219) 16.65%, rgb(190, 75, 219) 33.3%, rgb(76, 110, 245) 33.3%, rgb(76, 110, 245) 49.95%, rgb(64, 192, 87) 49.95%, rgb(64, 192, 87) 66.6%, rgb(250, 176, 5) 66.6%, rgb(250, 176, 5) 83.25%, rgb(253, 126, 20) 83.25%, rgb(253, 126, 20) 100%, rgb(250, 82, 82) 100%);
  animation: var(--timing) linear dance6123 infinite;
  transform: scale(1.1) translateY(-1px);
}

@keyframes dance6123 {
  to {
    background-position: var(--width);
  }
}