This page has a natural minimum width, created by 400px worth of side BODY padding. Go ahead and narrow the window, and at 400px wide you will see a horizontal scrollbar appear.
This effect is highly reliable across all modern browsers, but is really only needed for IE browsers, both Win (IE6 and lower) and Mac (IE5) since they do not support the min-width CSS property itself.
Before this method, the IE browsers needed a JavaScript (or some other type of scripting) which forced the browser to simulate min-width. However, some JavaScripts available display bugs in very complex served pages. This natural method of simulating min-width does not use any scripts.
Both .expander and .innerwrapper span the full width of the window, so they are available to receive any combination of background images and colors you like. This can be very handy for creating "faux columns" and other full-height effects.