ecco, ho modificato il tuo codice che aveva qualche errore, adesso con FF e safari mi pare vada bene, prova a controllare con IE (che non ce l'ho adesso sul Mac.
codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento senza titolo</title>
<style type="text/css">
<!--
.pippo {width:440px;
position:relative;
background-color:#666666;
margin:20px 0px; padding-left:135px;
padding-right:155px;
}
h3 {
font-family:Verdana, Arial, Helvetica, sans-serif;
font-size:15px; text-align:center;
margin:0px;
}
.pappo {width:46%;
height:100px;
padding:2%;
margin:10px 0px;
float:left;
background-color:#0000FF;
}
.pappone { width:46%;
height:100px; padding:2%;
margin:10px 0px;
float:right;
background-color:#00FFFF;
}
h4 { font-family:Times New Roman;
font-size:20px;
font-variant:small-caps;
color:#0000FF; width:100%;
//display:block;
margin:0px;
margin-bottom:5px;
padding:5px 0px;
border-bottom:1px solid #000000;
border-top:1px solid #000000;
background-color:#00FF00 ;}
-->
</style>
</head>
<body>
<div class="pippo"><h3>ciaO</h3>
<div class="pappo"></div>
<div class="pappone"></div>
<h4>novità</h4>
</div></body>
</html>