An interesting discovery for me recently (explaining part of the inconsistencies between IE and FF displays):
For firefox (and the W3C), the width of a box exclude the padding and the border.
For IE till 7, it includes it by default.
So if you set for example both width and padding to a block element, it will renderer differently in both browser.
However, I just learnt that you can turn it to “standard compliant mode” by defining the proper doctype on top of your files, e.g.:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
More info:
http://msdn.microsoft.com/en-us/library/bb250395(VS.85).aspx
