Can a td be inside a td

It may be a nonsense question, and I know we have to follow standards as much as possible. But can <td> be a direct child of another <td>, like this:

<td class="parent">
    <td class="child">
        <!-- Some info -->
    </td>
</td>

Or it's obligatory to create another <table> with a new <tr> before adding the <td>, which can become heavily populated with table tags and become clustered...