Template:AnimatedRainbowText/animation.css

 @keyframes rainbow{
   0%{color: red;}
   14%{color: orange;}
   28%{color: yellow;}
   42%{color: lime;}
   56%{color: cyan;}
   70%{color: blue;}
   84%{color: #FF00FF;}
   100%{color: red;}
 }
 
 .rainbow {
        animation: 5s rainbow 
 infinite;
 }