.div { 
    width: 246px;
    height: 70px;
    transition: transform 2s;
  }
  
  .div:hover {
    transform: rotate(360deg);
    background: 
    linear-gradient(rgba(255,0,0,1) 0%, rgba(255,154,0,1) 10%, rgba(208,222,33,1) 20%, rgba(79,220,74,1) 30%, rgba(63,218,216,1) 40%, rgba(47,201,226,1) 50%, rgba(28,127,238,1) 60%, rgba(95,21,242,1) 70%, rgba(186,12,248,1) 80%, rgba(251,7,217,1) 90%, rgba(255,0,0,1) 100%) 
    0 0/100% 200%;
    animation: a 1s linear infinite;
 }

 @keyframes a {
   to {background-position:0 -200%}
 }