.avatar {
  background: linear-gradient(    
      to right,
      rgba(255, 255, 255, 0),
      rgba(255, 255, 255, 0.5) 50%,
      rgba(255, 255, 255, 0) 80%
    ),
    lightgray;
  background-repeat: repeat-y;
  background-size: 500px 500px;
  background-position: 0 0;
  animation: shine 1s infinite;
}

@keyframes shine {  
  to {
    background-position: 100% 0, /* move highlight to right */ 0 0;
  }
}