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/md053.mjs

@@ -0,0 +1,60 @@
+// @ts-check
+
+import { addError, ellipsify } from "../helpers/helpers.cjs";
+import { getReferenceLinkImageData } from "./cache.mjs";
+
+const linkReferenceDefinitionRe = /^ {0,3}\[([^\]]*[^\\])\]:/;
+
+/** @type {import("./markdownlint.mjs").Rule} */
+export default {
+  "names": [ "MD053", "link-image-reference-definitions" ],
+  "description": "Link and image reference definitions should be needed",
+  "tags": [ "images", "links" ],
+  "parser": "none",
+  "function": function MD053(params, onError) {
+    const ignored = new Set(params.config.ignored_definitions || [ "//" ]);
+    const lines = params.lines;
+    const { references, shortcuts, definitions, duplicateDefinitions } =
+      getReferenceLinkImageData();
+    const singleLineDefinition = (line) => (
+      line.replace(linkReferenceDefinitionRe, "").trim().length > 0
+    );
+    const deleteFixInfo = {
+      "deleteCount": -1
+    };
+    // Look for unused link references (unreferenced by any link/image)
+    for (const definition of definitions.entries()) {
+      const [ label, [ lineIndex ] ] = definition;
+      if (
+        !ignored.has(label) &&
+        !references.has(label) &&
+        !shortcuts.has(label)
+      ) {
+        const line = lines[lineIndex];
+        addError(
+          onError,
+          lineIndex + 1,
+          `Unused link or image reference definition: "${label}"`,
+          ellipsify(line),
+          [ 1, line.length ],
+          singleLineDefinition(line) ? deleteFixInfo : undefined
+        );
+      }
+    }
+    // Look for duplicate link references (defined more than once)
+    for (const duplicateDefinition of duplicateDefinitions) {
+      const [ label, lineIndex ] = duplicateDefinition;
+      if (!ignored.has(label)) {
+        const line = lines[lineIndex];
+        addError(
+          onError,
+          lineIndex + 1,
+          `Duplicate link or image reference definition: "${label}"`,
+          ellipsify(line),
+          [ 1, line.length ],
+          singleLineDefinition(line) ? deleteFixInfo : undefined
+        );
+      }
+    }
+  }
+};