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

In the CSS2 font model, as in CSS1, each user agent has a "font database" at
its disposition. CSS1 referred to this database but gave no details about what
was in it. CSS2 defines the information in that database and allows style sheet
authors to contribute to it. When asked to display a character with a particular
font, the user agent first identifies the font in the database that "best fits" the
specified font (according to the font matching algorithm) [p. 231] Once it has
identified a font, it retrieves the font data locally or from the Web, and may
display the character using those glyphs.
In light of this model, we have organized the specification into two sections.
The first concerns the font specification mechanism [p. 197] , whereby authors
specify which fonts they would like to have used. The second concerns the font
selection mechanism [p. 212] , whereby the client’s user agent identifies and
loads a font that best fits the author’s specification.
How the user agent constructs the font database lies outside the scope of this
specification since the database’s implementation depends on such factors as
the operating system, the windowing system, and the client.
15.2 Font specification
The first phase of the CSS font mechanism concerns how style sheet authors
specify which fonts should be used by a user agent. At first, it seem that the
obvious way to specify a font is by it’s name, a single string - which appears to be
separated into distinct parts; for example "BT Swiss 721 Heavy Italic".
Unfortunately, there exists no well-defined and universally accepted taxonomy
for classifying fonts based on their names, and terms that apply to one font family
name may not be appropriate for others. For example, the term ’italic’ is
commonly used to label slanted text, but slanted text may also be labeled
Oblique, Slanted, Incline, Cursive, or Kursiv. Similarly, font names typically
contain terms that describe the "weight" of a font. The primary role of these
names is to distinguish faces of differing darkness within a single font family.
There is no accepted, universal meaning to these weight names and usage
varies widely. For example a font that you might think of as being bold might be
described as being Regular, Roman, Book, Medium, Semi- or Demi-Bold, Bold,
or Black, depending on how black the "normal" face of the font is within the
design.
This lack of systematic naming makes it impossible, in the general case, to
generate a modified font face name that differs in a particular way, such as being
bolder.
Because of this, CSS uses a different model [p. 198] . Fonts are requested not
through a single font name but through setting a series of font properties. These
property values form the basis of the user agent’s font selection [p. 212] mechanism.
The font properties can be individually modified, for example to increase
the boldness, and the new set of font property values will then be used to select
from the font database again. The result is an increase in regularity for style
sheet authors and implementors, and an increase in robustness.
197