gridview 每行的边框样式在ie6里不行


   <style type="text/css">
   .gdv_b{border-bottom:1px dashed #DDDDDD;}
   </style>
我用这个css控制每行的下边框样式，可是IE6里不行，Firefox里却可以，请问是为什么？只要有办法能让IE里也行就给分。下面是我的gridview
            <asp:GridView ID="GridView_BianJiTuiJian_YuanChuang" runat="server" DataSourceID="SqlDataSource_BianJiTuiJian_YuanChuang" 
                AutoGenerateColumns="False" DataKeyNames="S_ID" BackColor="White" 
                BorderColor="#CCCCCC" BorderStyle="None" BorderWidth="1px" CellPadding="4" ForeColor="Black" GridLines="None" 
                ShowHeader="False" RowStyle-CssClass="gdv_b" >
                    <Columns>
                        <asp:BoundField DataField="S_ID" HeaderText="S_ID" InsertVisible="False" ReadOnly="True" Visible="false"
                            SortExpression="S_ID" />
                        <asp:TemplateField><ItemTemplate><asp:CheckBox  ID="ckbox_tuijianYuanchuang" runat="server"/></ItemTemplate></asp:TemplateField>
                        <asp:BoundField DataField="S_Name" HeaderText="S_Name" SortExpression="S_Name" ItemStyle-HorizontalAlign="left" />
                        <asp:BoundField DataField="S_YanChang" HeaderText="S_YanChang" SortExpression="S_YanChang" ItemStyle-HorizontalAlign="left" />
                    </Columns>
                    <FooterStyle BackColor="#CCCC99" ForeColor="Black" />
                    <PagerStyle BackColor="White" ForeColor="Black" HorizontalAlign="Right" />
                    <SelectedRowStyle BackColor="#CC3333" Font-Bold="True" ForeColor="White" />
                    <HeaderStyle BackColor="#333333" Font-Bold="True" ForeColor="White" />
                </asp:GridView>