Add data-defines on well-known patterns of term/definition association

As discussed in speced#4522
See also w3c/reffy#1444

src/core/dfn.js

@@ -237,4 +237,26 @@ function addContractDefaults() {
   for (const dfn of exportableDfns) {
     dfn.dataset.export = "";
   }
+
+  // - Sets data-defines on well-known definition content patterns
+
+  // A dt with class dfn-desc (on in a dl with such a class)
+  // containing a definition
+  // indicates that the following dd or div element contains its prose content
+  for (const dt of document.querySelectorAll("dl.dfn-desc dt:has(dfn[data-dfn-type]), dt.dfn-desc:has(dfn[data-dfn-type])")) {
+    const dfnId = dt.querySelector("dfn[id]").id;
+    const dfnContent = dt.nextElementSibling;
+    if (dfnId && ["DIV", "DD"].includes(dfnContent.tagName)) {
+      dfnContent.dataset.defines = `#${dfnId}`;
+    }
+  }
+
+  // a non-dt element with class dfn-desc containing a definition
+  // indicates that the said element contains its prose content
+  for (const el of document.querySelectorAll(":not(dt):not(dl).dfn-desc:has(dfn[data-dfn-type])")) {
+    const dfnId = el.querySelector("dfn[id]").id;
+    if (dfnId) {
+      el.dataset.defines = `#${dfnId}`;
+    }
+  }
 }

tests/spec/core/dfn-spec.js

@@ -655,5 +655,33 @@ describe("Core — Definitions", () => {
       expect(errors[0].message).toContain("Declares both");
       expect(errors[1].message).toContain("but also has a");
     });
+
+    it("assigns data-defines on well-known pattern", async () => {
+      const body = `
+        <section>
+          <h2>Definition and its description</h2>
+          <p id="desc1" class="dfn-desc">
+            A <dfn>definition</dfn> can also have an associated description
+          </p>
+          <dl class="dfn-desc">
+            <dt><dfn>different convention</dfn></dt>
+            <dd id="desc2">Different conventions can be applied to associate a term with its description</dd>
+          </dl>
+          <dl>
+            <dt class="dfn-desc"><dfn>local convention</dfn></dt>
+            <dd id="desc3">The local convention can be applied to a dt individually</dd>
+          </dl>
+        </section>
+      `;
+      const ops = makeStandardOps(null, body);
+      const doc = await makeRSDoc(ops);
+      const desc1 = doc.getElementById("desc1");
+      const desc2 = doc.getElementById("desc2");
+      const desc3 = doc.getElementById("desc3");
+      expect(desc1.dataset.defines).toBe("#dfn-definition");
+      expect(desc2.dataset.defines).toBe("#dfn-different-convention");
+      expect(desc3.dataset.defines).toBe("#dfn-local-convention");
+    });
+
   });
 });