I disagree, here's a JS free solution, which works:
<html style="height: 100%;">
<body style="vertical-align: middle; margin: 0px; height: 100%;">
<div style="height: 100%; width: 100%; display: table; background-color: #ccc;">
<div style="display: table-cell; width: 100%; vertical-align: middle;">
<div style="height: 300px; width: 600px; background-color: wheat; margin-left: auto; margin-right: auto;">A</div>
</div>
</div>
</body>
</html>