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=utf-8" />
<script language="javascript">
function percentuale(importo,ali)
{
var imponibile = eval(importo)
var aliquota =  ali == "" ? 0 : eval(ali)
lordo = ((imponibile * aliquota)/100)+ imponibile;
document.getElementById("member_id").innerHTML=lordo;
}
</script>
<title>Documento senza titolo</title>
</head>
<body>
<form name="form">
Quando cambio campo lui scrive nel div


Imponibile netto:<input type="text" name="imponibile" maxlength="9" style="text-align:right;" onchange="percentuale(this.value,document.getElementById('aliquota').value);" id="imponibile" /> €

Aliquota IVA:
<input type="text" name="IVA" maxlength="9" style="text-align:right;" onchange="percentuale(document.getElementById('imponibile').value,this.value);" id="aliquota" /> % <span style="font-weight:bold;border:solid 1px#FFFF4D;">Lordo <span id="member_id"></span> €</span></td></tr>
</form>
</body>
</html>