[css-fonts-4] Move font-feature-settings from CSS Fonts 4 w3c#1653

Author: svgeesus

css-fonts-4/Overview.bs

@@ -4717,7 +4717,155 @@ to their initial values.</p>
 or <span title="font-variation-settings!!property">'font-variation-settings!!property'</span>.</p>
 
 <h3 id="font-feature-settings-prop">
-Low-level font feature settings control: the 'font-feature-settings' property</h3>
+Low-level font feature settings control: the 'font-feature-settings!!property' property</h3>
+
+
+<pre class="propdef">
+Name: font-feature-settings
+
+Value: 	normal | <<feature-tag-value>> #
+Initial: normal
+Applies to: all elements
+Inherited: yes
+Percentages: n/a
+Computed value: as specified
+Animation type: discrete
+</pre>
+
+<p>This property provides
+low-level control over OpenType font features.
+It is intended as a way of providing access
+to font features
+that are not widely used
+but are needed for a particular use case.
+
+<p class="advisement">Authors should generally
+use <span title="font-variant!!property">'font-variant!!property'</span>
+and its related subproperties
+whenever possible
+and only use this property
+for special cases
+where its use is the only way
+of accessing
+a particular infrequently used font feature.</p>
+
+<div class="example">
+<pre>/* enable small caps and use second swash alternate */
+font-feature-settings: "smcp", "swsh" 2;</pre>
+</div>
+
+<p>A value of
+<dfn id="font-feature-settings-normal-value" dfn-for="font-feature-settings" dfn-type="value"
+title="normal!!font-feature-settings">''font-feature-settings/normal''</dfn>
+means that no change
+in glyph selection or positioning
+occurs due to this property.</p>
+
+<p>Feature tag values have the following syntax:</p>
+
+<pre class="prod"><dfn dfn-for="font-feature-settings" dfn-type="value" id="feature-tag-value"><var>&lt;feature-tag-value&gt;</var></dfn> = &lt;string&gt; [ &lt;integer&gt; | on | off ]?</pre>
+
+<p>The &lt;string&gt; is a case-sensitive OpenType feature tag.
+As specified in the OpenType specification [[!OPENTYPE]],
+feature tags contain four ASCII characters.
+Tag strings longer or shorter than four characters,
+or containing characters outside the U+20&ndash;7E codepoint range
+are invalid.
+<!-- and what happens? ignored?
+	 they won't be found in the font anyway, so the next section applies -->
+Feature tags need only match
+a feature tag defined in the font,
+so they are not limited to
+explicitly registered OpenType features.
+Fonts defining custom feature tags
+should follow the
+<a href="https://www.microsoft.com/typography/otspec/featuretags.htm">tag name rules</a>
+defined in the OpenType specification [[!OPENTYPE-FEATURES]].
+
+<p id="must-not-synthesize-features">Feature tags
+not present in the font are ignored;
+a user agent must not attempt to synthesize fallback behavior
+based on these feature tags.
+The one exception is that
+user agents may synthetically support the <span class="tag">kern</span> feature
+with fonts that contain kerning data in the form of a 'kern' table
+but lack <span class="tag">kern</span> feature support
+in the 'GPOS' table.</p>
+
+<p class="note">In general, authors should
+use the 'font-kerning' property
+to explicitly enable or disable kerning
+since this property always affects fonts
+with either type of kerning data.</p>
+
+<p>If present,
+a value indicates an index used for glyph selection.
+An &lt;integer&gt; value must be 0 or greater.
+A value of 0 indicates that the feature is disabled.
+For boolean features,
+a value of 1 enables the feature.
+For non-boolean features,
+a value of 1 or greater enables the feature
+and indicates the feature selection index. A
+value of 'on' is synonymous with 1
+and 'off' is synonymous with 0.
+If the value is omitted, a value of 1 is assumed.
+</p>
+
+<p id="ffs-map">The computed value
+of font-feature-settings is a map,
+so any duplicates in the specified value
+must not be preserved.
+If the same axis name appears more than once,
+the value associated with the last appearance supersedes
+any previous value for that axis.</p>
+
+<div class="example">
+<pre>
+font-feature-settings: "dlig" 1;       /* dlig=1 enable discretionary ligatures */
+font-feature-settings: "smcp" on;      /* smcp=1 enable small caps */
+font-feature-settings: 'c2sc';         /* c2sc=1 enable caps to small caps */
+font-feature-settings: "liga" off;     /* liga=0 no common ligatures */
+font-feature-settings: "tnum", 'hist'; /* tnum=1, hist=1 enable tabular numbers and historical forms */
+font-feature-settings: "tnum" "hist";  /* invalid, need a comma-delimited list */
+font-feature-settings: "silly" off;    /* invalid, tag too long */
+font-feature-settings: "PKRN";         /* PKRN=1 enable custom feature */
+font-feature-settings: dlig;           /* invalid, tag must be a string */
+</pre>
+</div>
+
+<p>When values greater than the range
+supported by the font
+are specified,
+the behavior is explicitly undefined.
+For boolean features, in general these will enable the feature.
+For non-boolean features,
+out of range values will in general
+be equivalent to a 0 value.
+However,
+in both cases the exact behavior
+will depend upon the way the font is designed
+(specifically, which type of lookup is used
+to define the feature).</p>
+
+<p>Although specifically defined for OpenType feature tags,
+feature tags for other modern font formats
+that support font features
+may be added in the future.
+Where possible,
+features defined for other font formats
+should attempt to follow
+the pattern of registered OpenType tags.</p>
+
+<div class="example">
+<p>The Japanese text below will be rendered with half-width kana characters:</p>
+
+<pre lang="ja">
+body { font-feature-settings: "hwid"; /* Half-width OpenType feature */ }
+
+&lt;p>毎日<a href="https://example.com/?q=%E3%82%AB%E3%83%AC%E3%83%BC" style="text-decoration: none;">カレー</a>食べてるのに、飽きない&lt;/p>
+</pre>
+</div>
 
 
 <h3 id="font-language-override-prop">Font language override: the 'font-language-override' property</h3>
@@ -5879,6 +6027,7 @@ Changes
 <!-- new ones at the top -->
 
 <ul>
+	<li>Port font-feature-settings property from CSS Fonts 3</li>
 	<li>Port font-variant shorthand property from CSS Fonts 3</li>
 	<li>Port font-variant-east-asian property from CSS Fonts 3</li>
 	<li>Port font-variant-numeric property from CSS Fonts 3</li>