Untitled

<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Documento senza titolo</title>
</head>
<body>
<script type="text/javascript" language="javascript">

function checkform()
{
    var f = document.forms["theform"].elements;
    var cansubmit = true;

    for (var i = 0; i < f.length; i++)
	   {
		if (f[i].value.length == 0)
	     cansubmit = false;
	   }

    if (cansubmit)
	  {document.getElementById('submitbutton').disabled = false; }
    else
	   {document.getElementById('submitbutton').disabled = 'disabled'; }
}

function reset_campi()
{
 document.getElementById("theform").reset();
}

</script>

<form name="theform" id="theform">
Username:
<input type="text" onKeyup="checkform()" /><br><br>
Password:
<input type="text" onKeyup="checkform()" /><br><br>

<input type="button" id="submitbutton" value="Invio" disabled="disabled" onclick="reset_campi()"/>

<input type="button" onclick="reset_campi()" value="Reset form">

</form>
</body>
</html>