[Also sent to www-html-editor]

It is fairly common to sort tabular data by multiple columns --
implicitly creating nested rowgroups.

Neither colgroup nor any of the rowgroup instantiations (tbody, thead,
tfoot) can nest.

In the past, this was needed for backwards compatibility -- colgroup
elements were implicitly closed by the next colgroup.  In XHTML 2.0,
all elements will have closing tags, so it is possible to distinguish
a nested colgroup from an adjacent one.  Given this, I believe nested
colgroups and (rowgroups) should be permitted.

-jJ