[css-fonts] Copy over Glyph selection and positioning to L4

Author: svgeesus

css-fonts-4/Overview.bs

@@ -2648,12 +2648,62 @@ Font matching examples</h3>
 <h2 id="font-rend-props">
 Font Feature Properties</h2>
 
+Modern font technologies support a variety of advanced typographic
+and language-specific font features.  Using these features, a single
+font can provide glyphs for a wide range of ligatures, contextual and
+stylistic alternates, tabular and old-style figures, small capitals,
+automatic fractions, swashes, and alternates specific to a given
+language. To allow authors control over these font capabilities, the
+'font-variant' property has been expanded for CSS3. It now functions as
+a shorthand for a set of properties that provide control over
+stylistic font features.
+
 Issue: Import these sections from level 3, once it goes to Proposed Rec;
 stubs added as link targets for font tests moved to Level 4
 
 <h3 id="glyph-selection-positioning">
 Glyph selection and positioning</h3>
 
+This section is non-normative
+
+Simple fonts used for displaying Latin text use a very basic
+processing model. Fonts contain a <em>character map</em> which maps each
+character to a glyph for that character.  Glyphs for subsequent
+characters are simply placed one after the other along a run of text.  Modern font
+formats such as OpenType and AAT (Apple Advanced Typography) use a
+richer processing model. The glyph for a given character can be
+chosen and positioned not just based on the codepoint of the character
+itself, but also on adjacent characters as well as the language,
+script, and features enabled for the text.  Font features may be
+required for specific scripts, or recommended as enabled by default or
+they might be stylistic features meant to be used under author
+control.
+
+For a good visual overview of these features, see the
+[[OPENTYPE-FONT-GUIDE]].  For a detailed description of glyph processing for
+OpenType fonts, see [[WINDOWS-GLYPH-PROC]].
+
+Stylistic font features can be classified into two broad
+categories: ones that affect the harmonization of glyph shapes with the
+surrounding context, such as kerning and ligature features, and ones
+such as the small-caps, subscript/superscript and alternate features
+that affect shape selection.
+
+The subproperties of 'font-variant' listed below are used to control
+these stylistic font features. They do not control features that are
+required for displaying certain scripts, such as the OpenType features
+used when displaying Arabic or Indic language text.  They affect glyph
+selection and positioning, but do not affect font selection as
+described in the font matching section (except in cases required for
+compatibility with CSS 2.1).
+
+To assure consistent behavior across user agents, the equivalent
+OpenType property settings are listed for individual properties and
+are normative.  When using other font formats these
+should be used as a guideline to map CSS font feature property values
+to specific font features.
+
+
 <h3 id="language-specific-support">
 Language-specific display</h3>