Validate generated HTML.

.github/workflows/antsibull-docs.yml

@@ -34,7 +34,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          python -m pip install . coverage codecov
+          python -m pip install . coverage codecov html5lib
 
       - name: Use antsibull-docs sphinx-init
         run: |
@@ -59,6 +59,10 @@ jobs:
         run: |
           ./build.sh
 
+      - name: Validate HTML
+        run:
+          python .github/workflows/validate-html.py build/html/
+
       - name: Combine and upload coverage stats
         run: |
           coverage combine .coverage.*

.github/workflows/validate-html.py

@@ -0,0 +1,26 @@
+#!/usr/bin/python3
+
+import os
+import sys
+
+import html5lib
+
+
+errors = 0
+for path in sys.argv[1:]:
+    for dirname, _, files in os.walk(path):
+        for file in files:
+            if not file.endswith('.html'):
+                continue
+            path = os.path.join(dirname, file)
+            try:
+                parser = html5lib.HTMLParser(strict=True)
+                with open(path, 'rb') as f:
+                    document = parser.parse(f)
+            except Exception as e:
+                errors += 1
+                print(f'{path}: {e}')
+
+if errors > 0:
+    print(f'Found {errors} errors!')
+    sys.exit(1)