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/md001.mjs

@@ -0,0 +1,28 @@
+// @ts-check
+
+import { addErrorDetailIf } from "../helpers/helpers.cjs";
+import { getHeadingLevel } from "../helpers/micromark-helpers.cjs";
+import { filterByTypesCached } from "./cache.mjs";
+
+/** @type {import("./markdownlint.mjs").Rule} */
+export default {
+  "names": [ "MD001", "heading-increment" ],
+  "description": "Heading levels should only increment by one level at a time",
+  "tags": [ "headings" ],
+  "parser": "micromark",
+  "function": function MD001(params, onError) {
+    let prevLevel = Number.MAX_SAFE_INTEGER;
+    for (const heading of filterByTypesCached([ "atxHeading", "setextHeading" ])) {
+      const level = getHeadingLevel(heading);
+      if (level > prevLevel) {
+        addErrorDetailIf(
+          onError,
+          heading.startLine,
+          `h${prevLevel + 1}`,
+          `h${level}`
+        );
+      }
+      prevLevel = level;
+    }
+  }
+};