"em" should be horizontal, "ex" vertical
> > > The default size, the 100% size, and the 1em size should be the same,
> > > but they are not.
> > Why is this true? An em-space isn't necessarily the same as the font
> > height.
> In CSS1 it is. This is arguably underspecified, but section 6.1 reads:
> "ems, the height of the element's font"
> This usage of the term "em" is supported by  which reads:
> Em: In current usage, a unit of measurement that equals the point
> size of the type.
>  Glossary of typesetting terms, University of Chicago Press, 1994
While that's a good definition, typesetting practice /never/ uses "em"
as vertical measurement; an em is a horizontal measurement only, and an
"ex" is the vertical measurement. This makes measurements based on
fonts size work with compressed an/or fat fonts.
Lee Daniel Crocker <firstname.lastname@example.org> <http://www.piclab.com/lcrocker.html>
"All inventions or works of authorship original to me, herein and past,
are placed irrevocably in the public domain, and may be used or modified
for any purpose, without permission, attribution, or notification."--LDC