Actually, the behavior you are experiencing is the right box model behavior, which was "mistaken" in the old versions of Internet Explorer. Unfortunately, this is not a bug, but a correct layout. In brief, the height and width of the correct box model consists of width of the element, its border width and its padding. If an object is high and wide 100%, has a 5 pixels border and 10 pixels padding, its offsetWidth and offsetHeight are a sum of these 100% + its padding + its border width. The 100% is still 100%, but not of its CSS width and height, but its offsetWidth and offsetHeight.
The layout you are trying to create cannot be achieved with pure CSS.
Here is an article about the box model from W3C
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