"It is very aggravating on large displays, or for people with imperfect vision"
But not specifying a font size is annoying to people with average resolution displays and good vision because the default size (16px) is too large.
But I understand now how makecheck could get annoyed: though pixels can be resized after page load, ems and percents take the browser's font size setting into account and are scaled appropriately the first time.
It's possible for the user to change browser settings; I have mine set to 14pt. Should a Web designer assume that the user has configured the browser to their liking or not? Should this assumption depend on what decade it is?