html {
  margin:0;
  padding:0;
  background: url(nuage.jpg) no-repeat center fixed;
  -webkit-background-size: cover; /* pour Chrome et Safari */
  -moz-background-size: cover; /* pour Firefox */
  -o-background-size: cover; /* pour Opera */
  background-size: cover; /* version standardisée */
}
.container {
    font-family: "Arial Black", "Arial Bold", Gadget, sans-serif; font-size: 23px; 
    position: absolute;
    top: 50%;
    left: 50%;
    -moz-transform: translateX(-50%) translateY(-50%);
    -webkit-transform: translateX(-50%) translateY(-50%);
    transform: translateX(-50%) translateY(-50%);
}