html, body
{ width:100%; height:100%; margin:0; padding:0; }

div.aussentable
{ width:100%; height:100%; margin:0; padding:0; display:table; }

div.aussentd
{ width:100%; height:100%; margin:0; padding:0; display:table-cell; vertical-align:middle; }

div.innen
{ width:30em; margin-left:auto; margin-right:auto; text-align:left; }

<body>
<div class="aussentable">
<div class="aussentd">
<div class="innen">
<h1>Überschrift</h1>
<p>Inhalt ...</p>
</div>
</div>
</div>
</body>