What exactly changes in the css rendering, when desktop browsers zoom in or out on a website?

Question

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 (px, em, rem, etc).

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By the way, I am mainly concerned with zooming behaviour for modern desktop browsers. (I suspect mobile browser to be a straight enlargement of the whole page after rendering it normally, and know that old fashioned browsers just increment the base font-size). What isn't clear however, is what modern browsers (say the latest versions of Chrome or FF) do exactly when the user presses Ctrl + or Ctrl -.

Do they also just render the page normally (i.e. at 100%) and then just enlarge the rendered image? Because FF still seems to respect % widths for example, so it doesn't seem to be a straight up enlargement.

Answer 1

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