文本框默认值，当鼠标放上时变换

      <script type="text/javascript">
     function aa(obj)
     {
        obj.value = "";
     }
//     function bb(obj)
//     {
//        obj.value = "--请输入您要查询的域名--";onblur="bb(this);"
//     }
   </script>

 

 <asp:TextBox ID="TextBox1" runat="server" Text="--请输入您要查询的域名--" onfocus="aa(this);"  CssClass="searchtxt" Style="float: left"
                                Width="177px"></asp:TextBox><div style="float: right">
                                    <asp:ImageButton ID="ImageButton1" runat="server" ImageUrl="~/image/shb.jpg"
                                        onclick="ImageButton1_Click"  />