[css extraction] Handle subsidiary at-rules (#1377)

The `@font-feature-values` at-rule can contain subsidiary at-rules, defined in
the spec as at-rules scoped to `@font-feature-values`:
https://drafts.csswg.org/css-fonts/#font-feature-values-syntax

Reffy was not expecting at-rules to be scoped and extracted them as root
at-rules, with a `for` attribute that did not pass schema validation.

With this update, Reffy no longer lists subsidiary at-rules at main at-rules,
and rather lists them as descriptors under the main at-rule they are scoped to.

src/browserlib/extract-cssdfn.mjs

@@ -30,7 +30,7 @@ export default function () {
     // Note some selectors are re-defined locally in HTML and Fullscreen. We
     // won't import them.
     atrules: extractDfns({
-      selector: 'dfn[data-dfn-type=at-rule]',
+      selector: 'dfn[data-dfn-type=at-rule]:not([data-dfn-for])',
       extractor: extractTypedDfn,
       duplicates: 'reject',
       warnings
@@ -90,7 +90,18 @@ export default function () {
     });
   }
 
-  // Move descriptors to at-rules structure
+  // Subsidiary at-rules are at-rules that can be used within a parent at-rule,
+  // we'll consider that they are "descriptors".
+  const subsidiary = extractDfns({
+    selector: 'dfn[data-dfn-type=at-rule][data-dfn-for]',
+    extractor: extractTypedDfn,
+    duplicates: 'reject',
+    keepDfnType: true,
+    warnings
+  });
+  descriptors = descriptors.concat([subsidiary]);
+
+  // Move descriptors, and subsidiary at-rules, to at-rules structure
   for (const desclist of descriptors) {
     for (const desc of desclist) {
       let rule = res.atrules.find(r => r.name === desc.for);

tests/extract-css.js

@@ -437,6 +437,34 @@ const tests = [
   },
 
 
+  {
+    title: "understands subsidiary at-rules",
+    html: `
+      <pre class="prod">
+        @font-feature-values = &lt;family-name&gt;# { &lt;declaration-rule-list&gt; }
+      </pre>
+      <p><dfn data-dfn-type="at-rule" data-export="" id="at-ruledef-font-feature-values">@font-feature-values</dfn></p>
+      <pre class="prod">
+        <dfn data-dfn-for="@font-feature-values" data-dfn-type="at-rule" data-export="" id="at-ruledef-font-feature-values-stylistic">@stylistic</dfn> = @stylistic { &lt;declaration-list&gt; }
+      </pre>
+    `,
+    propertyName: "atrules",
+    css: [{
+      name: "@font-feature-values",
+      prose: "@font-feature-values",
+      value: "<family-name># { <declaration-rule-list> }",
+      descriptors: [
+        {
+          for: "@font-feature-values",
+          name: "@stylistic",
+          type: "at-rule",
+          value: "@stylistic { <declaration-list> }"
+        }
+      ]
+    }]
+  },
+
+
   {
     title: "ignores definitions that describe changes",
     html: `<table class="propdef">