Popup Login Box

#popupbox{
margin: 0;
margin-left: 40%;
margin-right: 40%;
margin-top: 50px;
padding-top: 10px;
width: 20%;
height: 150px;
position: absolute;
background: #FBFBF0;
border: solid #000000 2px;
z-index: 9;
font-family: arial;
visibility: hidden;}

<script language="JavaScript" type="text/javascript">
function login(showhide){
if(showhide == "show"){
document.getElementById('popupbox').style.visibility="visible";
}else if(showhide == "hide"){
document.getElementById('popupbox').style.visibility="hidden";
}
}
</script>

<div id="popupbox">
<form name="login" action="Versteckte-Seite.htm" method="post">
<center>Benutzername:</center> <center><input type="text" name="modulenter[name]" size="14" /></center>
<center>Passwort:</center> <center><input type="password" name="modulenter[kennwort]" size="14"/></center>
<center>
<input type="hidden" name="mode" value="1">
<input type="hidden" name="checklogin" value="1">
<input type="submit" value="Einloggen">
</center>
</form>
<br />
<center><a href="javascript:login('hide');">schließen</a></center> </div>
<p><a href="javascript:login('show');">Anmelden</a></p>