[blog] Lint duplicate h1 on the page (mui#40835)

.markdownlint-cli2.cjs

@@ -2,6 +2,7 @@ const straightQuotes = require('./packages/markdownlint-rule-mui/straight-quotes
 const gitDiff = require('./packages/markdownlint-rule-mui/git-diff');
 const tableAlignment = require('./packages/markdownlint-rule-mui/table-alignment');
 const terminalLanguage = require('./packages/markdownlint-rule-mui/terminal-language');
+const duplicateH1 = require('./packages/markdownlint-rule-mui/duplicate-h1');
 
 // https://github.com/DavidAnson/markdownlint#rules--aliases
 module.exports = {
@@ -35,8 +36,9 @@ module.exports = {
     gitDiff: true,
     tableAlignment: true,
     terminalLanguage: true,
+    duplicateH1: true,
   },
-  customRules: [straightQuotes, gitDiff, tableAlignment, terminalLanguage],
+  customRules: [straightQuotes, gitDiff, tableAlignment, terminalLanguage, duplicateH1],
   ignores: [
     'CHANGELOG.old.md',
     '**/node_modules/**',

packages/markdownlint-rule-mui/duplicate-h1.js

@@ -0,0 +1,31 @@
+// This rule is an extension of MD025/no-multiple-top-level-headings.
+// The rule is buggy https://github.com/DavidAnson/markdownlint/pull/1109
+// but also blog headers don't tell you that h1 is already injected.
+module.exports = {
+  names: ['duplicateH1'],
+  description: 'Multiple top-level headings in the same document.',
+  tags: ['headings'],
+  function: (params, onError) => {
+    let hasTopLevelHeading = false;
+    params.tokens.forEach((token) => {
+      if (token.type === 'heading_open' && token.tag === 'h1') {
+        // Avoid duplicate errors with MD025.
+        if (hasTopLevelHeading !== false && hasTopLevelHeading !== 1) {
+          onError({
+            lineNumber: token.lineNumber,
+          });
+        } else if (params.name.includes('/docs/pages/blog/')) {
+          onError({
+            lineNumber: token.lineNumber,
+            details: 'In the blog, the h1 is already added using the markdown header.title value.',
+          });
+        }
+
+        // Store the first h1 of the page.
+        if (hasTopLevelHeading === false) {
+          hasTopLevelHeading = token.lineNumber;
+        }
+      }
+    });
+  },
+};