I've been playing with the background properties and specifically size. One question that I can't find the answer to:
When background size is set to cover, what determines how large the image scales up to?
By definition, when the size is set to "cover":
"Scale the background image to be as large as possible so that the background area is completely covered by the background image. Some parts of the background image may not be in view within the background positioning area."
So the image scales to cover the background area. What constitutes the "background area?" Is that the user's screen, the body width, row width, column width, or something else?
From the css specs
Scale the image, while preserving its intrinsic aspect ratio (if any), to the smallest size such that both its width and its height can completely cover the background positioning area.
See CSS Backgrounds and Borders Module Level 3.