h1{
  background-image: linear-gradient(45deg, red 0%, orange 25%, yellow 50%, green 75%, blue 100%);
    font-size: 30px;
    box-sizing: border-box;
    display: inline-block;
    background-clip: text;
  -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
  text-fill-color: transparent;
  animation: auto-change-bg 1s linear infinite;
}
@keyframes auto-change-bg {
  0% {
    background-image: linear-gradient(45deg, red 0%, orange 25%, yellow 50%, green 75%, blue 100%);
  }
  50% {
    background-image: linear-gradient(45deg, blue 0%, red 25%, orange 50%, yellow 75%, green 100%);
  }
  100% {
    background-image: linear-gradient(45deg, green 0%, blue 25%, red 50%, orange 75%, yellow 100%);
  }
}