It's a classic problem - when you have an empty table cell the browser doesn't render borders around it. There are also two well-known workarounds. One is to place an in the table cell; the other is to use the empty-cells:show CSS property.
Unfortunately both have drawbacks. is kind of ugly when it comes to selecting text and copy-pasting it. You get a lot of spaces where there shouldn't be any, perhaps even with an exotic Unicode character. empty-cells:show should address exactly this problem, but unfortunately it only works properly in IE starting with version 8 (and then only in standards-compliant mode). It can be made to work in other versions by also specifying border-collapse: collapse, but sometimes this is what is NOT desired. In my case I have a fairly complex table and it relies on border-collapse:separate and would otherwise create quite a messy CSS/HTML soup.
So what are other things that you might put in a table cell that would make IE draw the borders yet not be visible or copyable? For all other browsers the empty-cells:show already does the trick, so I really just need to fool IE.