Grazie, adesso funziona:

codice:
<html>

<head>
<script language="javascript" type="text/javascript">
<!--

function LunghezzaMax(commento)
{
if (commento.value.length > 255 || commento.value.length < 10)
 {
alert("Inserire nel campo \"\Commento\"\nminimo 10 caratteri ed al massimo 255 caratteri.");

 }
}


function textCounter(field, countfield, maxlimit) {
if (field.value.length > maxlimit) 
field.value = field.value.substring(0, maxlimit);
else 
countfield.value = maxlimit - field.value.length;
}

//-->
</script>


</head>

<body>

<form method="POST" action="">
	


	
<textarea name="commento" rows="5" cols="32" onchange="LunghezzaMax(this)"; onKeyDown="textCounter(this.form.commento,this.form.remLen1,255);" onKeyUp="textCounter(this.form.commento,this.form.remLen1,255);">
</textarea>	

<input readonly type=text name=remLen1 size=3 maxlength=3 value="255">
	
	<input type="submit" value="Invia" name="B1">

</form>

</body>

</html>