<table summary="Showing off column grouping">
<caption>Column Groups</caption>
<colgroup span="2" style="background-color:pink"> span attribute is ignored
  <col width="100" />           
  <col span="2" width="200" />
</colgroup>
<colgroup span="2" width="300" />
<tr><th>ABC</th><th>DEF</th><th>GHI</th><th>JKL</th><th>MNP</th></tr>
<tr><td>12</td><td>24</td><td>36</td><td>1242812428</td><td>1262012620</td></tr>
<tr><td>36</td><td>72</td><td>98</td><td>1442814428</td><td>1062010620</td></tr>
</tbody></table>

colgroup's width attribute can, in theory, have multiple values or 0* (minimal width) but not supported by anybody

You can make a mess of tables if you get the row and columns spans wrong