This is checked and works in all browsers.
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<style type="text/css">
html, body { margin: 0; padding: 0; height: 100%; }
#outer {height: 100%; overflow: hidden; position: relative; width: 100%;}
#outer[id] {display: table; position: static;}
#middle {position: absolute; top: 50%; width: 100%; text-align: center;}
#middle[id] {display: table-cell; vertical-align: middle; position: static;}
#inner {position: relative; top: -50%; text-align: left;}
#inner {margin-left: auto; margin-right: auto;}
#inner {width: 300px; } /* this width should be the width of the box you want centered */
</style>
</head>
<body>
<div id="outer">
<div id="middle">
<div id="inner">
centered
</div>
</div>
</div>
</body>
</html>