pixels are pixels. zooming has nothing to do with css. with your css anyway. and i know of css zoom declaration
also, remember the os accessibility zoom features? is the same. or the font size in a word doc? it will always be 12px or 16px even if the zoom is 50% or 450%.
web dev hasn't invented anything new yet. it may only appear new due to the hype. but all new is already old. just that is badly implemented