Q:

Selecting specific tr element in a table (CSS)

I'm trying to select the 5% tr, but my css selector does not seem to work.  I also have to write one for the 10% tr, which I expect will be simple after the 5% tr is solved.  My selector seems to work down to table, but I cannot get it to select the second tr.  What am I doing wrong? 
Code: 

body > form > div > table > tr:nth-child(2) > td:first-child {
  background-color: red
}
<form method="post">
  <div>
    <table cellpadding="0" align="center" cellspacing="0" border="0">
      <tr>
        <td colspan="3">
          &nbsp;
        </td>
      </tr>
      <tr>
        <td style="width: 5%">
          5%
        </td>
        <td>
          <table width="100%" border="0" cellspacing="0" cellpadding="0" style="height: 100%; border-color: #e0e0e0;">
            <tr>
              <td colspan="2"></td>
            </tr>
            <tr>
              <td width="10%" valign="top" style="height: 100%;">10%</td>

              <td valign="top" width="90%">
                My Content
              </td>
            </tr>
          </table>
      </tr>
    </table>
  </div>
</form>

A:

If you do NOT want to change with your HTML markup, meaning adding classes (which would be easier to target the TR and TD) you can use :nth-of-type
Snippet

tr:nth-of-type(2) {
  background: red
}
table table tr:nth-of-type(2) td:first-of-type{
  background: lightblue
}
<table cellpadding="0" align="center" cellspacing="0" border="0">
      <tr>
        <td colspan="3">
          &nbsp;
        </td>
      </tr>
      <tr>
        <td style="width: 5%">
          5%
        </td>
        <td>
          <table width="100%" border="0" cellspacing="0" cellpadding="0" style="height: 100%; border-color: #e0e0e0;">
            <tr>
              <td colspan="2"></td>
            </tr>
            <tr>
              <td width="10%" valign="top" style="height: 100%;">10%</td>

              <td valign="top" width="90%">
                My Content
              </td>
            </tr>
          </table>
      </tr>
    </table>

A:

Target with following CSS:
tr[width="10%"]{

}

