It's a fad nowadays, but if you remember the bad old days when the "zoom" function in browsers was useless on most pages, that was because the browsers did it "by the book": they only zoomed elements with relative sizes, px units were always pixels. Then browser makers just gave up and reimplemented the zoom function to work with "broken" pages too (I think Chrome did it first and the rest followed suit). Of course the advent of Hi-DPI screens probably also played a part...