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

’ascent’ (Descriptor)
Value:
Initial: undefined
Media: visual
This is the descriptor for the maximum unaccented height [p. 229] of the font. If
the value is undefined, the descriptor is not used for matching. If this descriptor is
used, the ’units-per-em’ [p. 218] descriptor must also be used.
’descent’ (Descriptor)
Value:
Initial: undefined
Media: visual
This is the descriptor for the Maximum unaccented depth [p. 229] of the font. If
the value is undefined, the descriptor is not used for matching. If this descriptor is
used, the ’units-per-em’ [p. 218] descriptor must also be used.
15.3.7 Descriptors for Synthesis: ’widths’, ’bbox’ and
’definition-src’
Synthesizing a font means, at minimum, matching the width metrics of the specified
font. Therefore, for synthesis, this metric information must be available. Similarly,
progressive rendering requires width metrics in order to avoid reflow of the
content when the actual font has been loaded. Although the following descriptors
are optional for a CSS2 definition, some are required if synthesizing (or
reflow-free progressive rendering) is desired by the author. Should the actual font
become available, the substitute should be replaced by the actual font. Any of
these descriptors that are present will be used to provide a better or faster
approximation of the intended font.
Of these descriptors, the most important are the ’widths’ descriptor and ’bbox’
which are used to prevent text reflow should the actual font become available. In
addition, the descriptors in the set of descriptors used for matching [p. 220] can
be used to provide a better synthesis of the actual font appearance.
’widths’ (Descriptor)
Value: [ ]? [ ]+ [,[ ]? ]+]
Initial: undefined
Media: visual
This is the descriptor for the glyph widths [p. 228] . The value is a
comma-separated list of values each followed by one or more glyph
widths. If this descriptor is used, the ’units-per-em’ [p. 218] descriptor must also
be used.
If the is omitted, a range of U+0-7FFFFFFF is assumed which
covers all characters and their glyphs. If not enough glyph widths are given, the
last in the list is replicated to cover that urange. If too many widths are provided,
222