how to limit the number of characters in textarea

<script language="javascript" type="text/javascript">
function limitText(limitField, limitCount, limitNum) {
    if (limitField.value.length > limitNum) {
        limitField.value = limitField.value.substring(0, limitNum);
        alert("your message exceeds the limit");
    } else {
        limitCount.value = limitNum - limitField.value.length;
    }
}
</script>

<form name="myform" action="">
<textarea id="t1" name="description" onpropertychange="limitText(this.form.description,this.form.countdown,10);"  OnInput="limitText(this.form.description,this.form.countdown,10);" onKeyDown="limitText(this.form.description,this.form.countdown,10);" rows="20" cols="40">
</textarea><br>
<font size="1">(Maximum characters: 10)<br>
You have <input readonly type="text" name="countdown" size="3" value="10"> characters left.</font>
</form>