asp.net textarea 隐藏边框

<style>      -- 样式
  .textarea
        {
            border-width: 0px;
            background-color: navy;
            color: #FFFFFF;
            overflow: hidden;
            t: expression(this.onkeyup=function(){change(this)});
        }
    </style>

    <script>
        function change(obj) {
            obj.style.height = obj.scrollHeight;
        }
    </script>  
<asp:TextBox runat="server" CssClass="textarea" BorderWidth="0" TextMode="MultiLine"
        Text="ddddd" />