This is a window created entirely by CSS!
Okay, so maybe this isn't the best example of this CSS effect. I actually threw this dumb demo together in about fifteen minutes, so cut me some slack.

So how does this work?
The basic premise is very similar to CSS_Trans, except here we have a non-fixed (scrolling) background for the main body, and a fixed (non-scrolling) background for the text-boxes. The end result is something that looks somewhat like looking out a window. Heck, this demonstration even has a bug trying to escape!
Basically, we first set up the BODY properties with a non-fixed background:
BODY { background: black url(wood.png) 0 0; }
Next, we set the background and color for the window-boxes:
div#content { background: #82b2da url(sky.png) 0 0 fixed; }
Note that here we do fix the background (so it does not scroll). This is important to ensure we get the appropriate parallax for our windows.