Rainbow Shift

.btn-rainbow-shift {
  display: inline-block;
  padding: 13px 28px;
  border: none;
  border-radius: 10px;
  background: linear-gradient(90deg, #ff004c, #ff8a00, #ffd300, #00d26a, #00a3ff, #7b2ff7, #ff004c);
  background-size: 400% 100%;
  color: #ffffff;
  font-family: inherit;
  font-size: 15px;
  font-weight: 700;
  text-shadow: 0 1px 2px rgba(0, 0, 0, 0.25);
  cursor: pointer;
  animation: btn-rainbow-shift-flow 6s linear infinite;
}
.btn-rainbow-shift:hover {
  animation-duration: 1.5s;
}
@keyframes btn-rainbow-shift-flow {
  to {
    background-position: 400% 0;
  }
}