They go in the heaven of pixels ;)
Just joking. Most likely different browsers deal with it in different ways.
The first approach coming in my mind, is to calculate the width of each area. Then round it to the nearest int. The rest of the pixels gets just left empty.
Another approach, could be filling the last area, whatever it is the width (with a small margin of errors).
Very interesting question: it could be nice to have the control over this behaviour with css indeed.
