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/md020.mjs

@@ -0,0 +1,68 @@
+// @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": [ "MD020", "no-missing-space-closed-atx" ],
+  "description": "No space inside hashes on closed atx style heading",
+  "tags": [ "headings", "atx_closed", "spaces" ],
+  "parser": "micromark",
+  "function": function MD020(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)) {
+        const match =
+          /^(#+)([ \t]*)([^# \t\\]|[^# \t][^#]*?[^# \t\\])([ \t]*)((?:\\#)?)(#+)(\s*)$/.exec(line);
+        if (match) {
+          const [
+            ,
+            leftHash,
+            { "length": leftSpaceLength },
+            content,
+            { "length": rightSpaceLength },
+            rightEscape,
+            rightHash,
+            { "length": trailSpaceLength }
+          ] = match;
+          const leftHashLength = leftHash.length;
+          const rightHashLength = rightHash.length;
+          const left = !leftSpaceLength;
+          const right = !rightSpaceLength || !!rightEscape;
+          const rightEscapeReplacement = rightEscape ? `${rightEscape} ` : "";
+          if (left || right) {
+            const range = left ?
+              [
+                1,
+                leftHashLength + 1
+              ] :
+              [
+                line.length - trailSpaceLength - rightHashLength,
+                rightHashLength + 1
+              ];
+            addErrorContext(
+              onError,
+              lineIndex + 1,
+              line.trim(),
+              left,
+              right,
+              range,
+              {
+                "editColumn": 1,
+                "deleteCount": line.length,
+                "insertText":
+                  `${leftHash} ${content} ${rightEscapeReplacement}${rightHash}`
+              }
+            );
+          }
+        }
+      }
+    }
+  }
+};