Just to mess with your mind: in media queries and in the root element’s font-size declaration, em and rem mean the same thing, being the browser’s default font size, which is 16px in most browsers by default, but customisable in some browsers and different by default in some environments (e.g. I seem to recall Kindle using 19px). I call this unit the “browser em”, or bem. No one else really talks about it. One thing I haven’t investigated and probably should: does this unit depend on the language the document is specified in? (Because you can define different default fonts and sizes for different languages in at least Firefox.)
I would guess it does matter about the language, but likely because as the language changes the font itself may actually change. And different fonts would have different relative font sizes. But like you, I haven't investigated it enough to make a definitive statement about it.
