@keyframes move-clouds-back {
    from {background-position:0 0;}
    to {background-position:10000px 0;}
}
@-webkit-keyframes move-clouds-back {
    from {background-position:0 0;}
    to {background-position:10000px 0;}
}
@-moz-keyframes move-clouds-back {
    from {background-position:0 0;}
    to {background-position:10000px 0;}
}
@-ms-keyframes move-clouds-back {
    from {background-position: 0;}
    to {background-position:10000px 0;}
}

.clouds {
  position:fixed;
  top:0;
  left:0;
  right:0;
  bottom:0;
  width:100%;
  height:100%;
  display:block;
}

.clouds{
   background:transparent url(http://www.script-tutorials.com/demos/360/images/clouds3.png) repeat top center;
    z-index:3;

  -moz-animation:move-clouds-back 200s linear infinite;
  -ms-animation:move-clouds-back 200s linear infinite;
  -o-animation:move-clouds-back 200s linear infinite;
  -webkit-animation:move-clouds-back 200s linear infinite;
  animation:move-clouds-back 200s linear infinite;
}