the second table in section 12, and in the sample document, demonstrates an explicit <colgroup> but it omits the <tbody> element, violating the rule given a line or two above.

 <table>
      <colgroup>
        <col style="background-color:silver"/>
        <col style="background-color:gray"/>
        <col style="background-color:yellow"/>
      </colgroup>
      <tr>


should be


 <table>
      <colgroup>
        <col style="background-color:silver"/>
        <col style="background-color:gray"/>
        <col style="background-color:yellow"/>
      </colgroup>
<tbody>
      <tr>


actually the first row of both tables would probably be better in thead rather than tbody, but that's unrelated to polyglot considerations.