This is a page looking at using widths within widths to alter the layout of a page. It uses divs to alter widths.
It also uses percentages so the you alter the width of something relative to it's parent element
This alters the width of the margins because the div it's contained in is a different width - however, it doesn't work particularly well in Netscape. This paragraph isn't in a div so it just has a 10% margin