title: What is a font's "EM box/EM unit" and where is it defined
body: <p>In the <a href="http://www.w3.org/TR/css3-fonts/#font-size-prop" rel="noreferrer">CSS3 CSS Fonts Module Level 3 (candidate Oct 13)</a>, there is a single reference to the "EM box" and one to the "EM unit" in section 2.3. The quote follows:</p>
  
  <blockquote>
    <p>[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.</p>
  </blockquote>
  
  <p>What is the "EM box" or the related "EM unit"?</p>
  
  <p>Browsers (and other user agents) should get that from somewhere, from where?</p>