CSS3 is going to include a new attribute called box-sizing so we can choose whether the width set on an element will include borders and padding or whether borders and paddings will be added to the width.

The difference is basically the difference between the IE5 and W3C box models. The default will still be the W3C way we're all used to where the padding and borders will be added to the width, it'll just be possible to opt into the other way when it makes things easier.