dice qualcosa del genere:

<html>
<head>
<title>Finta pop-up</title>
<style type="text/css">
body {background:gray;
color:white
}
a {color:aqua}
.normale {width:90%;
background:maroon;
margin-left:5%!important;
margin-left:10%;
border:2px solid silver;
}
#pop_up {width:40%;
position:absolute;
background:teal;
border:2px solid silver;
top:3em;
left:30%;
display:none;
}
#pop_up span {background:maroon;
color:aqua;
border:1px solid silver;
float:right;

}
#pop_up a {padding:1em;
font-size:2em;
}
#pop_up h1 {clear:both}
</style>
<script type="text/javascript">
function mostra(){
ciccio=document.getElementById('pop_up');
ciccio.style.display="block"
}
function cela(){
ciccio=document.getElementById('pop_up');
ciccio.style.display="none"
}

</script>
</head>

<body>

<div class="normale">
<h2>Clicca qui per non aprire una pop-up</h2>
</div>
<div id="pop_up"><span>x</span>
<h1>Non è che sia una pop-up, ma dovrebbe avvicinarcisitivisi</h1>
</div>
</body>
</html>