John Resig decided to do a little experiment to see how browsers handle sub-pixel problems in CSS. John wanted to see what happens when you have 4 floated divs, each with a width of 25% that were contained inside a parent element with a fixed width of 50 pixels. This poses a problem since the correct mathematical width of each floated div should be 12.5px and browsers typically can't handle sub-pixel rendering.
Here's what John found:
This is actually something I've wondered about for a long time, but never taken the time to research how the browsers handle it.
I'd definitely go take a look at John's post because it contains lots of good information and comments.