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/md043.mjs

@@ -0,0 +1,70 @@
+// @ts-check
+
+import { addErrorContext, addErrorDetailIf } 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": [ "MD043", "required-headings" ],
+  "description": "Required heading structure",
+  "tags": [ "headings" ],
+  "parser": "micromark",
+  "function": function MD043(params, onError) {
+    const requiredHeadings = params.config.headings;
+    if (!Array.isArray(requiredHeadings)) {
+      // Nothing to check; avoid doing any work
+      return;
+    }
+    const matchCase = params.config.match_case || false;
+    let i = 0;
+    let matchAny = false;
+    let hasError = false;
+    let anyHeadings = false;
+    const getExpected = () => requiredHeadings[i++] || "[None]";
+    const handleCase = (str) => (matchCase ? str : str.toLowerCase());
+    for (const heading of filterByTypesCached([ "atxHeading", "setextHeading" ])) {
+      if (!hasError) {
+        const headingText = getHeadingText(heading);
+        const headingLevel = getHeadingLevel(heading);
+        anyHeadings = true;
+        const actual = `${"".padEnd(headingLevel, "#")} ${headingText}`;
+        const expected = getExpected();
+        if (expected === "*") {
+          const nextExpected = getExpected();
+          if (handleCase(nextExpected) !== handleCase(actual)) {
+            matchAny = true;
+            i--;
+          }
+        } else if (expected === "+") {
+          matchAny = true;
+        } else if (handleCase(expected) === handleCase(actual)) {
+          matchAny = false;
+        } else if (matchAny) {
+          i--;
+        } else {
+          addErrorDetailIf(
+            onError,
+            heading.startLine,
+            expected,
+            actual
+          );
+          hasError = true;
+        }
+      }
+    }
+    const extraHeadings = requiredHeadings.length - i;
+    if (
+      !hasError &&
+      ((extraHeadings > 1) ||
+        ((extraHeadings === 1) && (requiredHeadings[i] !== "*"))) &&
+      (anyHeadings || !requiredHeadings.every((heading) => heading === "*"))
+    ) {
+      addErrorContext(
+        onError,
+        params.lines.length,
+        requiredHeadings[i]
+      );
+    }
+  }
+};