moving cloud by css
#sky {
overflow: hidden;
}
#clouds {
width: 200%;
height: 400px;
background-image: url('pic.jpg');
background-size: cover;
-webkit-animation: movingclouds 25s linear infinite;
-moz-animation: movingclouds 25s linear infinite;
-o-animation: movingclouds 25s linear infinite;
}
@keyframes movingclouds {
0% {margin-left: 0%;}
100% {margin-left: -100%;}
}