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>
    <title>Pagina senza titolo</title>

    <script language="javascript" type="text/javascript">
// <!CDATA[
window.onload = function()
{
    document.getElementById("Text1").focus();
    
    document.getElementById("Text1").onblur = somma;
    document.getElementById("Text2").onblur = somma;
}

function somma()
{
    var x = parseFloat( "0" + document.getElementById("Text1").value);
    var y = parseFloat( "0" + document.getElementById("Text2").value);
    var z = y - x;
    document.getElementById("div1").innerHTML = z;
}


// ]]>
    </script>
</head>
<body>
    <input id="Text1" type="text" />
    

    <input id="Text2" type="text" />
    <div id="div1"></div>
</body>
</html>