position: absolute; to remove elements from the normal flow; position them absolutely with respect to the containing block of their closest ancestor element which has its position set to something other than static.
View the Source: In this case
#part1 has a parent (
#content) which has position set to relative. Try removing the rule for
#content and see how the layout changes. None of these divs have any content in them, but it's easy to see how overlapping quickly becomes a problem!