Test: https://upload.wikimedia.org/wikipedia/commons/2/29/Sketch_of_Google_Chrome_icon_(2008).png (https://commons.wikimedia.org/wiki/File:Sketch_of_Google_Chrome_icon_(2008).png).

Mouse cursor
- outside page: dark grey page.
- outside image: black image background
- outside image, holding button: light grey background
- at image: white background
- at image, holding button: chessboard grid

Changelog
- 2015-10-26 updated selector since `body` is no longer only-child`. Sorry, it must have been broken for quite a long time, I just forgot to push local version here.
- 2017-02-09 chrome 56 introduced own native centering and (static) checkerboard background: updated the style to get "interactive dynamics" back.

For Firefox: http://freestyler.ws/style/42947/firefox-4-center-images-dynamic-background