cloud{position: fixed;
top:0;
animation: move 50s linear infinite;
z-index:1;
}
c1 { top: 50px; animation-duration: 40s; }
c2 { top: 100px; animation-duration: 60s; }
c3 { top: 150px; animation-duration: 55s; }
c4 { top: 200px; animation-duration: 70s; }
c5 { top: 250px; animation-duration: 50s; }
c6 { top: 300px; animation-duration: 65s; }
c7 { top: 350px; animation-duration: 45s; }
@keyframes move {
  from { transform: translateX(100vw); }
  to   { transform: translateX(-400px); }
}
content {
  position: relative;
  z-index: 2;
}