Are you sure? I thought you could embed an Office object into a cell
of an Excel spreadsheet, so why can't that Office object be another
Excel spreadsheet? Not much of an Office expert so maybe I'm wrong.
Anyway considering that Excel and HTML have different purposes (though
both are widely used for all sorts of strange and bizarre additional
purposes) the analogy is quite weak.
<table>
<table>
</table></table>
Is invalid.
Correct. You're missing the required sub-element of table.
<table>
<tbody>
</tbody></table>
is valid.
Incorrect. You're missing the required sub-element of tbody.
However, the first one has two tables and the second one has one
table. So you're not even comparing like with like.
<table><tbody><tr><td>
<table><tbody><tr><td>
</td></tr></tbody></table>
</td></tr></tbody></table>
is valid and (give or take the totally optional <tbody> tags) is what
is being discussed here.
The issue is whether one can think of tabular data that can contain
another table of data as the content of one of the data cells. I can
think of a few examples but most of them would be handled better by
writing the main table differently instead.
The authors of HTML either didn't consider this issue or did consider
it and decided that it was conceivable that a data table could contain
another data table, hence nested tables are allowed in HTML, which
sadly is one of the things that made tables so damn easy to abuse for
layout purposes.
Steve