Convert markdownlint library to an ECMAScript module, replace markdownlint-micromark with micromark, stop publishing (large) markdownlint-browser.js, see https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c for guidance.

lib/md052.mjs

@@ -0,0 +1,39 @@
+// @ts-check
+
+import { addError } from "../helpers/helpers.cjs";
+import { getReferenceLinkImageData } from "./cache.mjs";
+
+/** @type {import("./markdownlint.mjs").Rule} */
+export default {
+  "names": [ "MD052", "reference-links-images" ],
+  "description":
+    "Reference links and images should use a label that is defined",
+  "tags": [ "images", "links" ],
+  "parser": "none",
+  "function": function MD052(params, onError) {
+    const { config, lines } = params;
+    const shortcutSyntax = config.shortcut_syntax || false;
+    const { definitions, references, shortcuts } = getReferenceLinkImageData();
+    const entries = shortcutSyntax ?
+      [ ...references.entries(), ...shortcuts.entries() ] :
+      references.entries();
+    // Look for links/images that use an undefined link reference
+    for (const reference of entries) {
+      const [ label, datas ] = reference;
+      if (!definitions.has(label)) {
+        for (const data of datas) {
+          const [ lineIndex, index, length ] = data;
+          // Context will be incomplete if reporting for a multi-line link
+          const context = lines[lineIndex].slice(index, index + length);
+          addError(
+            onError,
+            lineIndex + 1,
+            `Missing link or image reference definition: "${label}"`,
+            context,
+            [ index + 1, context.length ]
+          );
+        }
+      }
+    }
+  }
+};