[Extract elements] Strengthen logic for permission-elements (#2005)

Via w3c/browser-specs#2107 (comment)

The `permission-elements` spec uses what looks like an element definition table
to describe "common behaviours" of elements defined later in the spec. This
needed to be added as an exception to the rule.

The spec also links directly to the DOM interface. That simple case wasn't
captured properly. The logic is now more sound in that it looks for an IDL
interface `dfn` or a link to an IDL interface, and only parses the text to
handle the HTML spec case where the link to the IDL interface is not qualified.

Note: checked with a full crawl, this does not produce changes to the extracted
elements so far, and allows to crawl `html-permissions` as well.

Author: tidoust

src/browserlib/extract-elements.mjs

@@ -36,7 +36,9 @@ export default function (spec) {
       const dfns = [...heading.querySelectorAll('dfn')];
       if (dfns.length === 0) {
         // Ignore the definition of "Custom elements" in HTML
-        if (getText(heading).match(/Core concepts/)) {
+        // and the definition of "Common behaviours" in permission-elements
+        if (getText(heading).match(/Core concepts/i) ||
+            getText(heading).match(/Common behaviours/i)) {
           return null;
         }
         throw new Error('No dfn found in heading element: ' + heading.textContent);
@@ -101,14 +103,19 @@ export default function (spec) {
               dd = dd.nextElementSibling;
             }
             if (dd) {
-              let match = dd.textContent.match(/^interface (.*?) /m);
-              if (match) {
-                res[prop] = match[1];
+              const interfaceEl = dd.querySelector(`
+                dfn[data-dfn-type=interface],
+                a[data-link-type=idl]
+              `);
+              if (interfaceEl) {
+                res[prop] = interfaceEl.textContent.trim();
               }
               else {
+                // The HTML spec does not flag links to interfaces with a
+                // data-link-type attribute.
                 // NB: The sub/sup table uses a plural form for "Use", hence
                 // the "?" for the final "s" in "Uses".
-                match = dd.textContent.match(/^Uses? (.*?)[,\.\s]/);
+                const match = dd.textContent.match(/^Uses? (.*?)[,\.\s]/);
                 if (match) {
                   res[prop] = match[1];
                 }