Tables parsing issue - with <tr> inside <tr> case

When working with code generated by Microsoft Outlook I found a case where DOMDocument based parser has no problem with specific code, but HTML5 parser does.

The minimal test case input is this:

<table id="t1">
  <tr>
    <td>
      <table id="t2">
        <tr>
        <tr>
          <td></td>
        </tr>
        </tr>
      </table>
    </td>
  </tr>
  <tr><td></td></tr>
</table>

Note the <tr> element as a child of another <tr>. This causes HTML5 parser to output:

<table id="t1">
  <tr>
    <td>
      <table id="t2">
        <tr></tr>
        <tr>
          <td></td>
        </tr>
      </table>
    </td>
  </tr>
</table>
<tr><td></td></tr>

Which obviously is invalid and causes the parent table to be "closed" before it should, leaving the next (here: last) tr element outside of the table.

Reference: https://github.com/roundcube/roundcubemail/issues/7356

Start by reproducing the minimal nested-table input in the HTML5 parser and compare its output with the expected table structure shown in the issue. Trace the parser’s handling of a encountered inside another , then add coverage for this case and confirm that the final remains inside the parent table.