W3C CSS2 Cascading Style Sheets, level 2 - instructional media + ...

The ideal solution would be to have a name that uniquely identifies each
collection of font data. This name does not exist in current practice for font data.
Fonts with the same face name can vary over a number of descriptors. Some of
these descriptors, such as different complements of glyphs in the font, may be
insignificant if the needed glyphs are in the font. Other descriptors, such as different
width metrics, make fonts with the same name incompatible. It does not
seem possible to define a rule that will always identify incompatibilities, but will
not prevent the use of a perfectly suitable local copy of the font data with a given
name. Therefore, only the range of ISO 10646 characters will be used to qualify
matches for the font face name.
Since a prime goal of the font face name in the font definition is to allow a user
agent to determine when there is a local copy of the specified font data, the font
face name must be a name that will be in all legitimate copies of the font data.
Otherwise, unnecessary Web traffic may be generated due to missed matches
for the local copy.
15.4.3 Coordinate units on the em square
Certain values, such as width metrics, are expressed in units that are relative to
an abstract square whose height is the intended distance between lines of type in
the same type size. This square is called the em square and it is the design grid
on which the glyph outlines are defined. The value of this descriptor specifies
how many units the EM square is divided into. Common values are for example
250 (Intellifont), 1000 (Type 1) and 2048 (TrueType, TrueType GX and Open-
Type).
If this value is not specified, it becomes impossible to know what any font
metrics mean. For example, one font has lowercase glyphs of height 450;
another has smaller ones of height 890! The numbers are actually fractions; the
first font has 450/1000 and the second has 890/2048 which is indeed smaller.
15.4.4 Central Baseline
This gives the position in the em square [p. 227] of the central baseline. The
central baseline is used by ideographic scripts for alignment, just as the bottom
baseline is used for Latin, Greek, and Cyrillic scripts.
15.4.5 Font Encoding
Either explicitly or implicitly, each font has a table associated with it, the font
encoding table, that tells what character each glyph represents. This table is also
referred to as an encoding vector.
In fact, many fonts contain several glyphs for the same character. Which of
those glyphs should be used depends either on the rules of the language, or on
the preference of the designer.
In Arabic, for example, all letters have four (or two) different shapes, depending
on whether the letter is used at the start of a word, in the middle, at the end,
or in isolation. It is the same character in all cases, and thus there is only one
character in the source document, but when printed, it looks different each time.
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