Tables which are used purely for layout (as opposed to data tables), should not contain th elements, which would make the table appear to be a data table.

Example

Wrong

<table><tr><th>Sidebar</th><th>Content</th><tr><td>Navigation</td><td>Stuff</td></tr></table>

Right

<table><tr><td>Sidebar</td><td>Content</td><tr><td>Navigation</td><td>Stuff</td></tr></table>