title: What exactly changes in the css rendering, when desktop browsers zoom in or out on a website?
body: <p>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 (<code>px</code>, <code>em</code>, <code>rem</code>, etc).</p>
  
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  <p>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 <code>Ctrl +</code> or <code>Ctrl -</code>.</p>
  
  <p>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.</p>