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/md018.mjs

@@ -0,0 +1,43 @@
+// @ts-check
+
+import { addErrorContext } from "../helpers/helpers.cjs";
+import { addRangeToSet } from "../helpers/micromark-helpers.cjs";
+import { filterByTypesCached } from "./cache.mjs";
+
+/** @type {import("./markdownlint.mjs").Rule} */
+export default {
+  "names": [ "MD018", "no-missing-space-atx" ],
+  "description": "No space after hash on atx style heading",
+  "tags": [ "headings", "atx", "spaces" ],
+  "parser": "micromark",
+  "function": function MD018(params, onError) {
+    const { lines } = params;
+    const ignoreBlockLineNumbers = new Set();
+    for (const ignoreBlock of filterByTypesCached([ "codeFenced", "codeIndented", "htmlFlow" ])) {
+      addRangeToSet(ignoreBlockLineNumbers, ignoreBlock.startLine, ignoreBlock.endLine);
+    }
+    for (const [ lineIndex, line ] of lines.entries()) {
+      if (
+        !ignoreBlockLineNumbers.has(lineIndex + 1) &&
+        /^#+[^# \t]/.test(line) &&
+        !/#\s*$/.test(line) &&
+        !line.startsWith("#️⃣")
+      ) {
+        // @ts-ignore
+        const hashCount = /^#+/.exec(line)[0].length;
+        addErrorContext(
+          onError,
+          lineIndex + 1,
+          line.trim(),
+          undefined,
+          undefined,
+          [ 1, hashCount + 1 ],
+          {
+            "editColumn": hashCount + 1,
+            "insertText": " "
+          }
+        );
+      }
+    }
+  }
+};