Fills background image on a website that covers the entire browser window at all times. Let's put some specifics on it:

Fills entire page with image, no white space

Scales image as needed

Retains image proportions (aspect ratio)

Image is centered on page

Does not cause scrollbars

As cross-browser compatible as possible

Isn't some fancy shenanigans like Flash

Works in:

IE7+ (could probably get in IE6 with a fixed position shim)

Most any other desktop browser

Credits to:

https://css-tricks.com/perfect-full-page-background-image/