fix(template-no-empty-headings): align aria-hidden handling with WAI-ARIA spec

Per WAI-ARIA 1.2 §6.6, `aria-hidden` has value type true/false/undefined
with default `undefined`. Per §8.5, missing or empty-string attribute
values resolve to the default. So a valueless `aria-hidden` is NOT
hidden per spec — only an explicit `"true"` (ASCII case-insensitive per
HTML enumerated-attribute rules) hides the element.

The earlier direction of this PR borrowed the HTML boolean-attribute
intuition (presence = truthy) from jsx-a11y. That's a peer-plugin
convention, not a spec mandate — aria-hidden is an enumerated ARIA
attribute, not a boolean HTML one. vue-a11y's heading-has-content
doesn't exempt aria-hidden headings at all; lit-a11y has the inverse
rule.

Behaviour now:
- Exempt (hidden): `aria-hidden="true"` / "TRUE" / "True", `{{true}}`,
  `{{"true"}}` / case-variants.
- Flag (NOT hidden per spec): valueless `<h1 aria-hidden>`, empty
  `<h1 aria-hidden="">`, `{{false}}`, `{{"false"}}`, `"false"`.

Author: johanrd

lib/rules/template-no-empty-headings.js

@@ -1,17 +1,15 @@
 const HEADINGS = new Set(['h1', 'h2', 'h3', 'h4', 'h5', 'h6']);
 
+// Per WAI-ARIA 1.2 §6.6 and §8.5 (https://www.w3.org/TR/wai-aria-1.2/#aria-hidden),
+// aria-hidden has value type true/false/undefined with DEFAULT `undefined`, and
+// missing / empty-string values resolve to that default. So a valueless
+// `aria-hidden` is NOT hidden per spec — only an explicit `"true"` (ASCII
+// case-insensitive, per the enumerated-attribute rules) hides the element.
 function isAriaHiddenTruthy(attr) {
-  if (!attr) {
+  const value = attr?.value;
+  if (!value) {
     return false;
   }
-  const value = attr.value;
-  // Valueless or empty-string attribute — <h1 aria-hidden />. Per HTML boolean
-  // attribute semantics (and jsx-a11y/vue-a11y convention), presence = truthy.
-  if (!value || (value.type === 'GlimmerTextNode' && value.chars === '')) {
-    return true;
-  }
-  // HTML attribute values compare ASCII-case-insensitively, so "TRUE"/"True"
-  // count as truthy.
   if (value.type === 'GlimmerTextNode') {
     return value.chars.toLowerCase() === 'true';
   }

tests/lib/rules/template-no-empty-headings.js

@@ -44,16 +44,8 @@ ruleTester.run('template-no-empty-headings', rule, {
     '<template><h2><@heading /></h2></template>',
     '<template><h3><ns.Heading /></h3></template>',
 
-    // aria-hidden as a boolean / valueless / empty / mustache attribute — all
-    // should exempt the heading (aligns with jsx-a11y / vue-a11y treatment of
-    // boolean HTML attributes).
-    '<template><h1 aria-hidden></h1></template>',
-    '<template><h1 aria-hidden=""></h1></template>',
     '<template><h1 aria-hidden={{true}}></h1></template>',
     '<template><h1 aria-hidden="true">Visible to sighted only</h1></template>',
-
-    // HTML attribute values are ASCII case-insensitive — "TRUE" / "True" /
-    // {{"TRUE"}} / {{"True"}} all count as truthy.
     '<template><h1 aria-hidden="TRUE"></h1></template>',
     '<template><h1 aria-hidden="True"></h1></template>',
     '<template><h1 aria-hidden={{"TRUE"}}></h1></template>',
@@ -148,7 +140,19 @@ ruleTester.run('template-no-empty-headings', rule, {
       errors: [{ messageId: 'emptyHeading' }],
     },
 
-    // Falsy mustache aria-hidden values do not exempt the heading.
+    // Valueless / empty aria-hidden resolves to the default `undefined` per
+    // WAI-ARIA §6.6 — the element is NOT hidden, so an otherwise-empty heading
+    // still flags.
+    {
+      code: '<template><h1 aria-hidden></h1></template>',
+      output: null,
+      errors: [{ messageId: 'emptyHeading' }],
+    },
+    {
+      code: '<template><h1 aria-hidden=""></h1></template>',
+      output: null,
+      errors: [{ messageId: 'emptyHeading' }],
+    },
     {
       code: '<template><h1 aria-hidden={{false}}></h1></template>',
       output: null,