> What happens when that fake-100px-but-actually-10em wide element is meant to coexist with something that is actually set in pixels? For instance, a 120px skyscraper ad? The things sized in fake-pixels-but-actually-ems will change proportionally with the user’s font size, but the things sized in real pixels will not. All of a sudden different elements on the page have different ideas about what scale they should be rendered at, and the layout falls apart.
This is technically also working as intended. When a user scales their preferred font size in their browser settings, their expectation is that the font sizes on pages will scale accordingly, not that every element will scale. The latter is what zoom accomplishes, but there's a reason why both zoom and preferred font sizes exist in browser settings.
In your example, the ad (or image, or whatever) should only be sized in rems/ems if it has text. For all other elements that aren't related to text, it makes more sense to size with pixels. If everything is sized in ems/rems, then scaling the preferred font size behaves identically to zoom. This is less than ideal because if I want to increase font sizes, and what you do in response is zoom the whole page, then there is less space for that same text to fit on the page because it competes with other scaled elements. So while I can read the text more easily because the glyphs are larger, I can read _less_ of the text within the same amount of space than if text were the only scaled element.
Also, at least in my experience, designers aren't the ones thinking in ems; they typically hand us Figma compositions that use pixels, and we translate to rems in our code base. Designers design for the base experience, and we are responsible for coding it in a way that scales proportionally/in a way that respects user preferences.
This is technically also working as intended. When a user scales their preferred font size in their browser settings, their expectation is that the font sizes on pages will scale accordingly, not that every element will scale. The latter is what zoom accomplishes, but there's a reason why both zoom and preferred font sizes exist in browser settings.
In your example, the ad (or image, or whatever) should only be sized in rems/ems if it has text. For all other elements that aren't related to text, it makes more sense to size with pixels. If everything is sized in ems/rems, then scaling the preferred font size behaves identically to zoom. This is less than ideal because if I want to increase font sizes, and what you do in response is zoom the whole page, then there is less space for that same text to fit on the page because it competes with other scaled elements. So while I can read the text more easily because the glyphs are larger, I can read _less_ of the text within the same amount of space than if text were the only scaled element.
Also, at least in my experience, designers aren't the ones thinking in ems; they typically hand us Figma compositions that use pixels, and we translate to rems in our code base. Designers design for the base experience, and we are responsible for coding it in a way that scales proportionally/in a way that respects user preferences.