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/md025.mjs

@@ -0,0 +1,37 @@
+// @ts-check
+
+import { addErrorContext, frontMatterHasTitle } from "../helpers/helpers.cjs";
+import { getHeadingLevel, getHeadingText } from "../helpers/micromark-helpers.cjs";
+import { filterByTypesCached } from "./cache.mjs";
+
+/** @type {import("./markdownlint.mjs").Rule} */
+export default {
+  "names": [ "MD025", "single-title", "single-h1" ],
+  "description": "Multiple top-level headings in the same document",
+  "tags": [ "headings" ],
+  "parser": "micromark",
+  "function": function MD025(params, onError) {
+    const level = Number(params.config.level || 1);
+    const foundFrontMatterTitle =
+      frontMatterHasTitle(
+        params.frontMatterLines,
+        params.config.front_matter_title
+      );
+    let hasTopLevelHeading = false;
+    for (const heading of filterByTypesCached([ "atxHeading", "setextHeading" ])) {
+      const headingLevel = getHeadingLevel(heading);
+      if (headingLevel === level) {
+        if (hasTopLevelHeading || foundFrontMatterTitle) {
+          const headingText = getHeadingText(heading);
+          addErrorContext(
+            onError,
+            heading.startLine,
+            headingText
+          );
+        } else if (heading.startLine === 1) {
+          hasTopLevelHeading = true;
+        }
+      }
+    }
+  }
+};