.chat  {  
  animation: marcherev 1s steps(12) infinite;
  background: url(../img/chatrev.png) 0 0 no-repeat; 
  height: 35px;
  width: 200px;  
}

 @keyframes marcherev {  
  0% {background-position: 0 0; } 
  100% {background-position: 0 -450px;}   
}