<!-- 需jquery支持 -->
<script>
    function count_content(){
        var num = $('name[name=content]').val().length;
        $('form[name=comment]').find('em').html(num.toString());
    }
</script>

<form name="comment">
    <textarea name="content" onKeyup="count_content()"></textarea>
    <em>0</em> / 140个字符
</form>