In what way is the design scaled up or down? I'm trying to figure out what exactly happens at the CSS level, and what the consequences are for different sizing methods (
Zooming as implemented in modern browsers consists of nothing more than “stretching up” pixels. That is, the width of the element is not changed from 128 to 256 pixels; instead the actual pixels are doubled in size. Formally, the element still has a width of 128 CSS pixels, even though it happens to take the space of 256 device pixels.
In other words, zooming to 200% makes one CSS pixel grow to four times the size of one device pixels. (Two times the width, two times the height, yields four times in total).
For detailed explanation you could read the following page: Concept of device pixels and CSS pixels