[css-fonts-4] allow font-display in @font-feature-values. Fix #2973

Author: svgeesus

css-fonts-4/Overview.bs

@@ -4963,7 +4963,7 @@ mapping author-chosen human-friendly names
 to feature indexes for the associated feature.
 
 <pre class=prod>
-@font-feature-values <<family-name>># { <<rule-list>> }
+@font-feature-values <<family-name>># { <<declaration-list>> }
 
 <<feature-value-block>> = <<font-feature-value-type>> { <<declaration-list>> }
 
@@ -4982,8 +4982,15 @@ If syntax errors occur within the <<family-name>> list,
 the entire rule ''@font-feature-values'' rule is invalid
 and must be ignored.
 
-The ''@font-feature-values'' block accepts <<feature-value-block>>s as its contents;
-these are [=at-rules=] named by one of the <<font-feature-value-type>> at-keyword tokens.
+The ''@font-feature-values'' block accepts <<declaration-list>> as its contents;
+these list items are either:
+
+[=at-rules=] named by one of the <<font-feature-value-type>> at-keyword tokens,
+or
+the '@font-feature-values/font-display' descriptor.
+
+<!-- TODO: tab says "The OM needs to make sure it exposes both .style and .childRules or whatever." -->
+
 Specifying the same <<font-feature-value-type>> more than once is valid;
 their contents are cascaded together.
 Each <<feature-value-block>> accepts a list of [=declarations=],
@@ -7203,6 +7210,7 @@ Changes
 
 <ul>
 	<li>Reduced the critical angle for oblique font matching from 20deg to 11deg, per WG resolution</li>
+	<li>Allowed font-display in @font-feature-values (the prose allowed it but the grammar did not)</li>
 </ul>
 
 <h3 id="changes-2020-11-17">Changes from the <a href="https://www.w3.org/TR/2020/WD-css-fonts-4-20201117/">17 November 2020 Working Draft</a> </h3><!-- See https://github.com/w3c/csswg-drafts/commits/main/css-fonts-4/Overview.bs -->