This example is a solution to the problem of a "floating" site conetnt box obscuring a static side bar when the browser window is resized to a smaller size.
Another and perhaps better solution is to use a single container box and give it "auto" left and right margins, and equal amounts of left padding and left placement (200px for example)
The CCS styles for this page are located in the <head> tag of this page, so just view the page source code to see what's going on.