Código HTML:
<html>
<head>
<title>Aprendiendo Javascript</title>
<style type='text/css'>
.mostrar {display:none}
</style>
<script language='javascript'>
var mostrar = document.getElementById("mostrar");
</script>
</head>
<body bgcolor='#000000'>
<a href='javascript:;' onclick='document.getElementById("mostrar").className=""'>Mostrar</a>
<table border='1' cellspacing='0'>
<tr>
<td>
<div id='mostrar' style='color:red;border:solid #cdcdcd
1.5px;top:50%;left:50%;position:absolute;width:200px;height:200px' class='mostrar'>
<div style='cursor:default;background-color:#CDCDCD;top:50%;width:15px;height:15px;'
onclick='document.getElementById("mostrar").className="mostrar"'>X</div>
Link : <input type='text' value=''/>
</div>
</td>
</tr>
<td>
<img width='100' height='100'/>
</td>
</tr>
</table>
</body>
</html>