@keyframes rainbow {
  0% { background-color: red; }
  14% { background-color: orange; }
  28% { background-color: yellow; }
  42% { background-color: green; }
  57% { background-color: blue; }
  71% { background-color: indigo; }
  85% { background-color: violet; }
  100% { background-color: red; } /* Loop back to start */
}

body {
  animation: rainbow 10s infinite;
  margin: 0;
  height: 100vh;
}