Informações do Cartão de Crédito Tipo do Cartão: MasterCard Visa * Número do Cartão: *

Com o fundo cinza claro está o RequiredFieldValidator e em azul claro está o campo ValidationSummary.

Código:
<table cellpadding="10">
                  <tr>
                     <td>
                        <table style="background-color:#eeeeee; padding:10px">
 
                           <tr>
                              <td colspan="3">
                                 <b>Informações do Cartão de Crédito</b>
                              </td>
                           </tr>
                           <tr>
                              <td align="right">
                                 Tipo do Cartão:
                              </td>
                              <td>
                                 <asp:RadioButtonList id="RadioButtonList1" RepeatLayout="Flow" runat="server">
                                    <asp:ListItem>MasterCard</asp:ListItem>
                                    <asp:ListItem>Visa</asp:ListItem>
                                 </asp:RadioButtonList>
                              </td>
                              <td align="center" rowspan="1">
                                 <asp:RequiredFieldValidator
                                      id="RequiredFieldValidator1"
                                      ControlToValidate="RadioButtonList1"
                                      ErrorMessage="Tipo do Cartão. "
                                      Display="Static"
                                      InitialValue=""
                                     Width="100%"
                                     runat="server">
                                    *
                                 </asp:RequiredFieldValidator>
                              </td>
                           </tr>
                           <tr>
                              <td align="right">
                                 Número do Cartão:
                              </td>
                              <td>
                                 <asp:TextBox id="TextBox1" runat="server" />
                              </td>
                              <td>
                                 <asp:RequiredFieldValidator
                                      id="RequiredFieldValidator2"
                                      ControlToValidate="TextBox1"
                                      ErrorMessage="Número do Cartão. "
                                      Display="Static"
                                      Width="100%"
                                     runat="server">
                                    *
                                 </asp:RequiredFieldValidator>
                              </td>
                           </tr>
 
                           <tr>
                              <td></td>
                              <td>
                                 <asp:Button id="Button1" text="Validar" runat="server" />
                              </td>
                              <td></td>
                           </tr>
                        </table>
                     </td>
                     <td valign="top">
                        <table cellpadding="20">
                           <tr>
                              <td>
                                 <asp:ValidationSummary
                                      id="valSum"
                                      DisplayMode="BulletList"
                                      runat="server"
                                      HeaderText="Você deve digitar o valor dos seguintes campos:"
                                      Font-Names="verdana"
                                      Font-Size="12"/>
                              </td>
                           </tr>
                        </table>
                     </td>
                  </tr>
               </table>