I have created a JSFiddle (containing only HTML and CSS) which shows my approach to the problem so far:
As you can see, the second cell in the master table contains an inner table. What I would like to be able to do is to resize this inner table (or maybe modify its enclosing cell) so that only the top and left strip of the enclosing cell is shown. The enclosing cell's text should remain, and the inner table should stay inside the enclosing cell as the picture in my original post shows.
Is there a way of doing this?