CSS
#grad{
background: linear-gradient(to right, red,orange,yellow,green,blue,indigo,violet);
-webkit-background-clip: text;
-webkit-text-fill-color: transparent;
font-size:100px;
animation: los 1s linear infinite;
}
@keyframes los{
0%{ background-image: linear-gradient(to right, red,orange,yellow,green,blue,indigo,violet);-webkit-background-clip: text;-webkit-text-fill-color: transparent;}
12,5%{ background-image: linear-gradient(to right, orange,yellow,green,blue,indigo,violet,red);-webkit-background-clip: text;-webkit-text-fill-color: transparent;}
25%{ background-image: linear-gradient(to right, yellow,green,blue,indigo,violet,red,orange);-webkit-background-clip: text;-webkit-text-fill-color: transparent;}
37,5%{ background-image: linear-gradient(to right, green,blue,indigo,violet,red,orange,yellow);-webkit-background-clip: text;-webkit-text-fill-color: transparent;}
50%{ background-image: linear-gradient(to right,blue,indigo,violet,red, orange,yellow,green);-webkit-background-clip: text;-webkit-text-fill-color: transparent;}
62,5%{ background-image: linear-gradient(to right,indigo,violet,red, orange,yellow,green,blue);-webkit-background-clip: text;-webkit-text-fill-color: transparent;}
75%{ background-image: linear-gradient(to right, violet,red,orange,yellow,green,blue,indigo);-webkit-background-clip: text;-webkit-text-fill-color: transparent;}
87,5%{ background-image: linear-gradient(to right, violet,red,orange,yellow,green,blue,indigo);-webkit-background-clip: text;-webkit-text-fill-color: transparent;}
100%{ background-image: linear-gradient(to right,red,orange,yellow,green,blue,indigo,violet);-webkit-background-clip: text;-webkit-text-fill-color: transparent;}
}