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

15.4.19 Vertical stem width
This is the width of vertical (or near-vertical) stems of glyphs. This information is
often tied to hinting, and may not be directly accessible in some font formats. The
measurement should be for the dominant vertical stem in the font because there
might be different groupings of vertical stems (e.g., one main one, and one lighter
weight one as for an uppercase M or N).
15.4.20 Vertical stroke angle
This is the angle, in degrees counterclockwise from the vertical, of the dominant
vertical strokes of the font. The value is negative for fonts that slope to the right,
as almost all italic fonts do. This descriptor may also be specified for oblique
fonts, slanted fonts, script fonts, and in general for any font whose vertical
strokes are not precisely vertical. A non-zero value does not of itself indicate an
italic font.
15.5 Font matching algorithm
This specification extends the algorithm given in the CSS1 specification. This
algorithm reduces down to the algorithm in the CSS1 specification when the
author and reader style sheets do not contain any @font-face rules.
Matching of descriptors to font faces must be done carefully. The descriptors
are matched in a well-defined order to insure that the results of this matching
process are as consistent as possible across UAs (assuming that the same
library of font faces and font descriptions is presented to each of them). This
algorithm may be optimized, provided that an implementation behaves as if the
algorithm had been followed exactly.
1. The user agent makes (or accesses) a database of relevant font-face
descriptors of all the fonts of which the UA is aware. If there are two fonts
with exactly the same descriptors, one of them is ignored. [p. 42] The UA
may be aware of a font because:
it has been installed locally
it is declared using an @font-face rule in one of the style sheets linked
to or contained in the current document
it is used in the UA default style sheet, which conceptually exists in all
UAs and is considered to have full @font-face rules for all fonts which
the UA will use for default presentation, plus @font-face rules for the
five special generic font families (see ’font-family’) defined in CSS2
2. At a given element and for each character in that element, the UA assembles
the font properties applicable to that element. Using the complete set of
properties, the UA uses the ’font-family’ descriptor to choose a tentative font
family. Thus, matching on a family name will succeed before matching on
some other descriptor. The remaining properties are tested against the
family according to the matching criteria described with each descriptor. If
there are matches for all the remaining properties, then that is the matching
font face for the given element.
3. If there is no matching font face within the ’font-family’ being processed by
step 2, UAs that implement intelligent matching may proceed to examine
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