div#mydiv {
margin: -10px;
}

* { margin: 0; padding: 0; }

<div id="shader"></div>

#shader {position: absolute; width: 100%; height: 100%; min-height: 100%; top: 0; left: 0;}

div.cover { position: fixed; top: 0px; left: 0px; right: 0px; bottom: 0px; }

<div style="position:absolute;left:0px;top:0px;width:100%;height:100%;">...

<div style="margin:-100%">

<style type="text/css">
    #superdiv{ position:fixed; top:0; left:0; width:100%; height:100%; }
</style>

<div id="superdiv">
    Put some content here.
</div>