how to export data to an excel by preserving styles

I copied your code and did the test. Result: no problem.

However, I used the following table data in my test. Did you use any data in yours? To fully test an HTML table, you have to put in some data.

<!--- <cfheader name="content-disposition" value="attachment; filename=dashboard.xls">

<cfcontent type="application/vnd.ms-excel"> --->
<table width="200" border="1">
  <tr>
    <td width="40">xxx</td>
    <td width="160">
      <table width="160" border="1">
      <tr>
        <td width="20" bgcolor="red">AAA</td><td width="20" bgcolor="black">ccc</td><td width="20" bgcolor="red">eee</td><td width="20" bgcolor="black">ggg</td>
        <td width="20" bgcolor="red">&n b s p;</td><td width="20" bgcolor="black">ddd</td><td width="20" bgcolor="red">fff</td><td width="20" bgcolor="black">hhh</td>
      </tr>
      </table>
    </td>
  </tr>
  <tr>
    <td width="40">yyy</td>
   <td width="160">
      <table width="160" border="1">
      <tr>
        <td width="20" bgcolor="red">111</td><td width="20" bgcolor="black">333</td><td width="20" bgcolor="red">555</td><td width="20" bgcolor="black">&n b s p;</td>
        <td width="20" bgcolor="red">222</td><td width="20" bgcolor="black">444</td><td width="20" bgcolor="red">666</td><td width="20" bgcolor="black">888</td>
      </tr>
      </table>
    </td>
  </tr>
</table>

Two other things to note. First, I have used &n b s p; for empty table-data. Secondly, though the MIME type you have used for excel might work, the correct  value is application/vnd.ms-excel.