#slideIn {
border: 1px solid red; 
  width: 50px;
  height: 40px;
  background: blue;
  position: relative;
  left: 500px;
  -webkit-animation: slideIn 2s forwards;
  -moz-animation: slideIn 2s forwards;
  animation: slideIn 2s forwards;
}
@-webkit-keyframes slideIn {
  0% {
    transform: translateX(-900px);
  }
  100% {
    transform: translateX(0);
  }
}
@-moz-keyframes slideIn {
  0% {
    transform: translateX(-900px);
  }
  100% {
    transform: translateX(0);
  }
}
@keyframes slideIn {
  0% {
    transform: translateX(-900px);
  }
  100% {
    transform: translateX(0);
  }
}