Table in table, after tr, but not td.
<br> the outter table and tr are ignored.

<table border>
<caption> caption of table2.</caption>
<tr>
  <table border>
  <caption> caption of table1.</caption>
  <tr>
  <td>first cell in table1
  <td>second cell in table1
  <tr>
  <td>third cell in table1
  <td>fourth cell in table1
  </table>
<tr>
<td>This is the first cell in table2.
</table>

NOTE, even no line break here! 
<br>
Caption of outer table is ignored.
Data of outer table is treated as text.