table显示细线边框

<table cellspacing="0" cellpadding="0" border="0" width="500">
        <tr>
            <td bgcolor="#AAAAAA">
                <table cellspacing="1" cellpadding="0" border="0" width="100%">
                    <tr bgcolor="#FFFFFF">
                        <td height="20">
                            &nbsp;
                        </td>
                        <td>
                            &nbsp;
                        </td>
                        <td>
                            &nbsp;
                        </td>
                    </tr>
                    <tr bgcolor="#FFFFFF">
                        <td height="20">
                            &nbsp;
                        </td>
                        <td>
                            &nbsp;
                        </td>
                        <td>
                            &nbsp;
                        </td>
                    </tr>
                </table>
            </td>
        </tr>
    </table>