Prova detta:
Kod:
<script type="text/javascript">
function closethis() {
document.getElementById("whatever").style.display="none";
}
</script>
<div id="whatever" style="position: absolute; left: 50%; top: 50%; margin-left: -250px; margin-top: -100px; z-index: 101; height: 200px; width: 500px; border: 1px solid #0000FF; background: #FFFFFF; ">
<a href="javascript:closethis()">Stäng</a>
</div>
Fungerar såvitt jag vet i IE5+, Firefox, Opera, m.fl.