CSS box model
W3C box model (width specifies pure content area width) represents bottom-up philosophy where content specifies the area for itself. Parent elements should adapt to it.
IE6 box model (width specifies content area with borders and paddings) represents top-down philosophy where designer specifies available spaces for nested elements.
In fact, content designers don’t care about the container width and html designers think better in terms of IE6 box model. Thanks to stupid standards, we all have to make additional calculations in our heads when working with styleshits.