Код:
<html>
<head>
<script type="text/javascript">
<!--
function prov(){
var sch = document.getElementById("mar").value.length;
if (sch>=255){
return false
}else{
return true
}
}
//-->
</script>
</head>
<body>
<form name="form1">
<input type="text" readonly="readonly" name="num"><br>
<textarea id="mar"
onkeypress="return prov()"
onkeyup="document.form1.num.value=255-this.value.length"
>
</textarea>
</body>
</html>