You know, like a tic-tac-toe board. I was just pondering how to do this the other day, as one does. There are three ways I can think of. One involves a good handful of rules and is the way I intuitively think of, one involves a deprecated attribute, and one is very simple and feels kinda like a CSS trick.
Possibility #1) Removing the Borders You Don't Need
This is the first way I think of. Add a border everywhere, then remove the border on the: