ho fatto un esempio giusto per sgranchire la tastiera e
codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>opacity e position</title>
<style type="text/css">
body {
	background-image: url(http://www.ilovecosta.it/style/bg.jpg);
}
.pag {
	width: 50%;
	position: relative;
	margin: 0 auto;
}
.sfondo {
	background-color: rgb(204,204,204);
	position: absolute;
	left: 0px;
	top: 0px;
	right: 0px;
	bottom: 0px;
	opacity: 0.85;
}
.testo {
	position: relative;
	z-index: 2;
}
</style>
</head>
<body>
<div class="pag">
  <div class="sfondo"></div>
  <div class="testo">
    

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  </div>
</div>
</body>
</html>