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

For example, here the font ’Robson Celtic’ is defined and referenced in a style
sheet contained in an HTML document.
Font test
@font-face {
font-family: "Robson Celtic";
src: url("http://site/fonts/rob-celt")
}
H1 { font-family: "Robson Celtic", serif }
This heading is displayed using Robson Celtic
The style sheet (in the STYLE element) contains a CSS rule that sets all H1
elements to use the ’Robson Celtic’ font family.
A CSS1 implementation will search the client for a font whose family name and
other properties match ’Robson Celtic’ and, if it fails to find it, will use the
UA-specific fallback serif font (which is defined to exist [p. 210] ).
A user agent implementing CSS2 will first examine @font-face rules in search
of a font description defining ’Robson Celtic’. This example contains a rule that
matches. Although this rule doesn’t contain much font data, it does have a URI
where the font can be retrieved for rendering this document. Downloaded fonts
should not be made available to other applications. If no matching @font-face is
found, the user agent will attempt the same match as a user agent implementing
CSS1.
Note that if the font ’Robson Celtic’ had been installed on the client system,
this would have caused the UA to add an entry in the font database for the
installed copy as described in the section on the font matching algorithm [p. 231]
. The installed copy would have been matched before the downloadable font in
the example above.
CSS1 implementations, which do not understand the @font-face rule, will
encounter the opening curly brackets and will ignore [p. 42] forward until the
matching closing curly brackets. This at-rule conforms with the forward-compatible
parsing [p. 35] requirement of CSS. Parsers may ignore [p. 42] these rules
without error.
Having the font descriptors separate from the font data has a benefit beyond
being able to do font selection and/or substitution. The data protection and replication
restrictions on the font descriptors may be much weaker than on the full
font data. Thus, it may be possible to install the font definition locally, or at least
to have it in a local cache if it occurs in a commonly referenced style sheet; this
would not require accessing the full font definition over the Web more than once
per named font.
If a font descriptor is duplicated, the last occurring descriptor wins and the rest
must be ignored.
Also, any descriptors that are not recognized or useful to the user agent must
be ignored [p. 42] . Future versions of CSS may allow additional descriptors for
the purpose of better font substitution, matching, or synthesis.
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