<div class="progress"></div>

.progress {
  width:120px;
  height:20px;
  -webkit-mask:linear-gradient(90deg,#000 70%,#0000 0) 0/20%;
  background:
   linear-gradient(#000 0 0) 0/0% no-repeat
   #ddd;
  animation:p 2s infinite steps(6);
}
@keyframes p {
    100% {background-size:120%}
}