I think there’s a misunderstanding about where the guidelines come from.
Alastair Campbell

Thanks for clarifying. It sounds like the point you’re getting at is that accessibility isn’t a point in favor of rem/em since pixels achieve the WCAG criteria just as well. I hear you on that, but if that means developers should be free to use pixels, where does that leave the font size option in browsers? Should devs treat it as deprecated or something? I’m not super comfortable with an approach like that, since it seems like it forces users with custom font sizes to switch to using zoom (unless I’m missing something).

Also, I don’t think the choice is between purely relying on zoom (i.e. using pixels) and emulating zoom (i.e. using ems/rems). There are ways to use a mix of both to account for different needs, especially now that we have things like calc(), Grid, and the upcoming min(), max(), and clamp(). I think Jen Simmons has talked about this a bit in a different context, the idea of mixing units in “intrinsic” web design.