the *precise* definition of 1em

I'm trying to figure out the *precise* definition of 1em so I can
implement it in E-scape correctly.

What if I write something like this:

BODY { font: numbers, someotherfont; font-size: 11pt }

Now, assume that numbers is a font that contains glyphs only for the ten
digits, and someotherfont contains all the glyphs.  Furthurmore, assume
that we don't have 11 point versions of either font, so we end up using
a 10 point version of numbers and a 12 point version of someotherfont.

Now, the CSS1 spec does state that if we can't accomodate the exact
value, we should approximate, and use the approximated value.  But
in this example, it's hard to know which of the two fonts to use.
(Note that I delibrately constructed the example so taht one could claim
`numbers is only used for a few glyphs; most glyphs come from someotherfont,
so use someotherfont for the size'.  or one could argue `numbers comes
first, so use that.')

but to make sure that writing font-size: 1em will be a no-op, I guess
we have to use 11pt, thus *not* approximating.  (It's also much easier
for me to implement it that way :)