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body {
  background: url(bg.jpg) no-repeat center center fixed;
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
  filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(src='.bg.jpg', sizingMethod='scale');
  -ms-filter: "progid:DXImageTransform.Microsoft.AlphaImageLoader(src='bg.jpg', sizingMethod='scale')";
}

body:before {
    content: "";
    display: block;
    position:fixed;
    width:100%;
    height:100%;
    background: url("bg.jpg") no-repeat;
    z-index:-1;
}