Template:AnimatedRainbowText/rainbow.css

@keyframes rainbow {
  0% { color: red; }
  16% { color: orange; }
  32% { color: yellow; }
  48% { color: lime; }
  64% { color: cyan; }
  80% { color: blue; }
  90% { color: #FF00FF; }
  100% { color: red; }
}

.rainbow {
  animation: 2s rainbow infinite;
  animation-timing-function: linear;
}