build(aio): fail build if more than h1 is found in a doc

Closes #16193

aio/tools/transforms/angular-base-package/index.js

@@ -127,7 +127,8 @@ module.exports = new Package('angular-base', [
     addImageDimensions.basePath = path.resolve(AIO_PATH, 'src');
     postProcessHtml.plugins = [
       require('./post-processors/autolink-headings'),
-      addImageDimensions
+      addImageDimensions,
+      require('./post-processors/h1-checker'),
     ];
   })
 

aio/tools/transforms/angular-base-package/post-processors/h1-checker.js

@@ -0,0 +1,18 @@
+const visit = require('unist-util-visit');
+const is = require('hast-util-is-element');
+const source = require('unist-util-source');
+
+module.exports = function h1CheckerPostProcessor() {
+  return (ast, file) => {
+    let h1s = [];
+    visit(ast, node => {
+      if (is(node, 'h1')) {
+        h1s.push(node);
+      }
+    });
+    if (h1s.length > 1) {
+      const h1Src = h1s.map(node => source(node, file)).join(', ');
+      file.fail(`More than one h1 found [${h1Src}]`);
+    }
+  };
+};

aio/tools/transforms/angular-base-package/post-processors/h1-checker.spec.js

@@ -0,0 +1,49 @@
+var testPackage = require('../../helpers/test-package');
+var Dgeni = require('dgeni');
+const plugin = require('./h1-checker');
+
+describe('h1Checker postprocessor', () => {
+  let processor, createDocMessage;
+
+  beforeEach(() => {
+    const dgeni = new Dgeni([testPackage('angular-base-package')]);
+    const injector = dgeni.configureInjector();
+    createDocMessage = injector.get('createDocMessage');
+    processor = injector.get('postProcessHtml');
+    processor.docTypes = ['a'];
+    processor.plugins = [plugin];
+  });
+
+  it('should complain if there is more than one h1 in a document', () => {
+    const doc = {
+      docType: 'a',
+      renderedContent: `
+        <h1>Heading 1</h2>
+        <h2>Heading 2</h2>
+        <h1>Heading 1a</h1>
+    `
+    };
+    expect(() => processor.$process([doc])).toThrowError(createDocMessage('More than one h1 found [<h1>Heading 1, <h1>Heading 1a</h1>]', doc));
+  });
+
+  it('should not complain if there is exactly one h1 in a document', () => {
+    const doc = {
+      docType: 'a',
+      renderedContent: `
+        <h1>Heading 1</h2>
+        <h2>Heading 2</h2>
+    `
+    };
+    expect(() => processor.$process([doc])).not.toThrow();
+  });
+
+  it('should not complain if there are no h1s in a document', () => {
+    const doc = {
+      docType: 'a',
+      renderedContent: `
+        <h2>Heading 2</h2>
+    `
+    };
+    expect(() => processor.$process([doc])).not.toThrow();
+  });
+});