My parent defines my width
Parents usually choose the food that their children eat which can have a significant effect on the child's weight and width of their waste line. Interesting thought, but what I'm talking about is a parent div and a child div.
Usually I setup my sites with an outer div around 960px wide. All divs inside the container are given a width of 100% by default by all browsers. Therefore I usually don't specify widths for the inside containers unless they are floated or positioned absolutely. If I have a div and need to specify the width, I'll set padding on the outer div that surrounds it, this way, each child div is controlled by its parent.
If the outer div is 940px wide and you would like the inner div to be 920px width, do not set the width of the inner div. Set a padding value on the outer div and the inner div will have less room to expand. Therefore to create a more robust page, let parent divs define a child divs width.