Using rem (default 16px), one can write web page containing html elements and texts in harmony. Default root element font size is 16px, but many users override it for better readability. Use of rem respects that.

https://www.sitepoint.com/understanding-and-using-rem-units-in-css/

Setting default font size and browser zoom level are two different controls. Browser zoom scales up everything by n times, e.g., css 10px becomes 20px when zoomed 2x.

https://www.quirksmode.org/mobile/viewports.html

OTOH, media query assumes default font size 16px and is not affected by font size override in css, e.g., font-size of body.