Do not report null href when algorithm dfn has no ID

Happens in CSS 2.1 (because the spec is old) and in HTML (because there are a
few remaining dfns without ID).

src/browserlib/extract-algorithms.mjs

@@ -371,8 +371,22 @@ function getDefinedNameIn(el) {
     else {
       name += getTextContent(dfn);
     }
-    const href = dfn.id ? getAbsoluteUrl(dfn) : null;
-    return { name, href };
+    if (dfn.id) {
+      return { name, href: getAbsoluteUrl(dfn) };
+    }
+    else {
+      // Two known exceptions to the rule:
+      // - one due to CSS 2.1 not following the definitions data model:
+      // https://www.w3.org/TR/CSS21/visudet.html#containing-block-details
+      // - the other due to HTML still containing dfns without IDs as well,
+      // including one for an algorithm:
+      // https://html.spec.whatwg.org/multipage/server-sent-events.html#processField
+      // It's possible to find an ID in both cases. But it's not clear that
+      // CSS 2.1 algorithms are real algorithms; and it seems doable to fix the
+      // HTML spec. Let's just return the name without href, not to end up
+      // with a null `href` that the JSON schema forbids.
+      return { name };
+    }
   }
   else {
     const heading = el.querySelector('h2[id],h3[id],h4[id],h5[id],h6[id]');

tests/extract-algorithms.js

@@ -367,6 +367,19 @@ const tests = [
     ]
   },
 
+  {
+    title: 'does not return a null href when dfn has no ID',
+    html: `
+      <p>To <dfn data-export="" data-dfn-type="dfn">do something</dfn>, just do something.</p>`,
+    algorithms: [
+      {
+        name: 'do something',
+        rationale: 'To <dfn>',
+        html: 'To <dfn data-export="" data-dfn-type="dfn">do something</dfn>, just do something.'
+      }
+    ]
+  },
+
 ];
 
 describe('The algorithms extraction module', function () {