lucy24 - 11:39 pm on Sep 18, 2012 (gmt 0)
You can't. The border is, by definition, outside whatever element it's a border of. Or, in the case of cells, between the present cell and the adjoining ones. So adding a border will always make the element bigger.
You can cheat by setting a default all-the-time border with the same color as your page background and the same width as your sometimes-visible border. You can say "transparent" and most browsers will humor you, but technically it's an error and won't validate. Borders are foreground, not background. You might also experiment by using padding instead of a border and changing its color. Padding is background, so it defaults to transparent. But it may not give the visual effect you want.