// Function

<script type="text/javascript">
    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>




// Textfield

<textarea id="userAbout" onkeyup="textCounter(this.form.userAbout, this.form.remLen, 300)"></textarea>

// Characters remaining

<input id="remLen" type="text" value="300" size="3" maxlength="3" readonly disabled/>