Add HTML validation rules for missing elements

See new rules:

* https://html-validate.org/rules/input-missing-label.html
* https://html-validate.org/rules/no-missing-references.html

packages/govuk-frontend/.htmlvalidate.js

@@ -15,9 +15,15 @@ module.exports = {
     // Allow pattern attribute on input type="number"
     'input-attributes': 'off',
 
+    // Require input to have a label
+    'input-missing-label': 'error',
+
     // Allow inline styles for testing purposes
     'no-inline-style': 'off',
 
+    // Require all form field and ARIA references to exist
+    'no-missing-references': 'error',
+
     // Allow for explicit roles on regions that have implict roles
     // We do this to better support AT with older versions of IE that
     // have partial support for HTML5 semantic elements