//The CSS
#horizon {position:absolute; top:50%; left:0px; width:100%; height:1px; overflow:visible;}
#content {position:absolute; top:-200px; left:50%; margin-left:-300px; width:600px; height:400px; background-color:#000;}
//The Html
<div id="horizon">
<div id="content">
</div>
</div>
