I hate IE with a passion when it comes to CSS. 5.5, 6, 7, and coming very soon 8 all seem to render css different. 8 at least is supposed to have a standards compliant mode. Yea, we shall see.
I know there is a border line type issue that can get you up to 2 px more than you wanted in FF. say you have a box
+-------+
| |
| |
| |
+-------+
FF will render from within the borders the num of pixels you want.
+-------+
| ^ |
|< + |
| |
+-------+
arrows pointing to the inside edge of our div container
In IE, they of course like to think different, ~*cough RETARDS*~ So they render from outside the object border (even if their is no border)
+-------+
| |
>| |
| |
+-------+
^
arrows pointing from outside into
the inside of our div container.
This small difference can add up to 2 px difference in many layouts if not accounted for. 2px is huge. Often a work around to have cross browser compatibility is to wrap the div inside a div. Set the height width and it forces what you would expect.