In the CSS3 CSS Fonts Module Level 3 (candidate Oct 13), there is a single reference to the "EM box" and one to the "EM unit" in section 2.3. The quote follows:
[The font-size] property indicates the desired height of glyphs from the font. For scalable fonts, the font-size is a scale factor applied to the EM unit of the font. (Note that certain glyphs may bleed outside their EM box.) For non-scalable fonts, the font-size is converted into absolute units and matched against the declared font-size of the font, using the same absolute coordinate space for both of the matched values.
What is the "EM box" or the related "EM unit"?
Browsers (and other user agents) should get that from somewhere, from where?