Media queries using relative units

How are media queries using relative units supposed to be computed? The draft says:

"The ‘em’ value is relative to the initial value of ‘font-size’."
<http://www.w3.org/TR/css3-mediaqueries/#width>

However, this statement is contained in an example. So it's not clear if this is meant to be a normative requirement or a fact that happens to be true for some other (non-obvious) reason, and it's not clear if it applies to all uses of relative units or only the specific example.

Regards,
Maciej