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

page can be displayed.
Progressive rendering is a combination of download and one of the other
methods; it provides a temporary substitute font (using name matching, intelligent
matching, or synthesis) to allow content to be read while the requested font
downloads. Once the real font has been successfully downloaded, it replaces the
temporary font, hopefully without the need to reflow.
Note. Progressive rendering requires metric information about the font in order
to avoid re-layout of the content when the actual font has been loaded and
rendered. This metric information is sufficiently verbose that it should only be
specified at most once per font in a document.
15.3.1 Font Descriptions and @font-face
The font description provides the bridge between an author’s font specification
and the font data, which is the data needed to format text and to render the
abstract glyphs to which the characters map - the actual scalable outlines or
bitmaps. Fonts are referenced by style sheet properties.
The font description is added to the font database and then used to select the
relevant font data. The font description contains descriptors such as the location
of the font data on the Web, and characterizations of that font data. The font
descriptors are also needed to match the style sheet font properties to particular
font data. The level of detail of a font description can vary from just the name of
the font up to a list of glyph widths.
Font descriptors may be classified into three types:
1. those that provide the link between the CSS usage of the font and the font
description (these have the same names as the corresponding CSS font
properties),
2. the URI for the location of the font data,
3. those that further characterize the font, to provide a link between the font
description and the font data.
All font descriptions are specified via a @font-face at-rule. The general form is:
@font-face { }
where the has the form:
descriptor: value;
descriptor: value;
[...]
descriptor: value;
Each @font-face rule specifies a value for every font descriptor, either implicitly
or explicitly. Those not given explicit values in the rule take the initial value listed
with each descriptor in this specification. These descriptors apply solely within
the context of the @font-face rule in which they are defined, and do not apply to
document language elements. Thus, there is no notion of which elements the
descriptors apply to, or whether the values are inherited by child elements.
The available font descriptors are described in later sections of this specification.
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