Despite some claims to the contrary, IE6/Win's rendering of this page is not correct, as it (as well as some other browsers) doesn't correctly support background-attachment: fixed for any element other than the body. That makes it impossible to pull off the intended effect. Other browsers may or may not get the effect right.
[...]
According to CSS, any background image that is "fixed" using background-attachment: fixed; is fixed with respect to the viewport-- not the element with which the image is associated. So I set the blue-shell background image to be aligned with the top left corner of the browser window (the viewport) with the values given for background-position. However, the image will only be visible wherever is intersects with the element to which it's been assigned. Therefore, even though the top left corner of the blue-shell image is aligned with the top left corner of the viewport, we can only see it wherever it intersects with a div that has an id with a value of content (which, again, happens to be the one containing this text).