fix(template-no-empty-headings): lean toward fewer positives on valueless aria-hidden

The valueless / empty-string aria-hidden case is genuinely contested in
the ecosystem — four positions exist (jsx-a11y / vue-a11y / axe-core /
WAI-ARIA spec), and no single authoritative source is decisive. Rather
than pick one interpretation and live with its false positives, this
rule leans toward fewer-false-positives: any aria-hidden form that could
plausibly mean "hide this" exempts the heading from the empty-content
check.

Truthy (exempt heading):
- valueless `<h1 aria-hidden>` — undefined-default per spec, but
  authors who write bare aria-hidden plausibly intend to hide.
- empty `<h1 aria-hidden="">` — same.
- `aria-hidden="true"` (ASCII case-insensitive) — unambiguous.
- `aria-hidden={{true}}` / `{{"true"}}` (case-insensitive) — unambiguous.

Falsy (still flag empty heading):
- `aria-hidden="false"`, `{{false}}`, `{{"false"}}` — explicit opt-out.

This reverses the previous spec-first direction on the valueless/empty
case. Rationale: a linter that flags intentional decorative markup
creates friction and loss of trust; a linter that misses some genuinely-
empty headings is preferable when the signal is ambiguous. The explicit
`aria-hidden="true"` cases, which ARE clearly hidden per spec, remain
exempt.

Author: johanrd

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

@@ -1,17 +1,35 @@
 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.
+// aria-hidden semantics for valueless / empty / "false" are genuinely
+// contested — four ecosystem positions exist (jsx-a11y / vue-a11y / axe /
+// WAI-ARIA spec), see PR body. This rule leans toward FEWER false positives:
+// when the author has written `aria-hidden` in any form that could plausibly
+// mean "hide this", we exempt the heading from the empty-content check. The
+// downside (missing some genuinely-empty headings) is preferable to flagging
+// correctly-authored headings the developer intentionally decorated.
+//
+// Truthy:
+//  - valueless attr (`<h1 aria-hidden>`) — default-undefined per spec, but
+//    authors who write bare `aria-hidden` plausibly intend hidden.
+//  - empty string `aria-hidden=""` — same.
+//  - `aria-hidden="true"` / "TRUE" / "True" (ASCII case-insensitive).
+//  - `aria-hidden={{true}}` mustache boolean literal.
+//  - `aria-hidden={{"true"}}` / case-variants as mustache string literal.
+// Not truthy (falls through):
+//  - `aria-hidden="false"` / `{{false}}` / `{{"false"}}` — explicit opt-out.
 function isAriaHiddenTruthy(attr) {
-  const value = attr?.value;
-  if (!value) {
+  if (!attr) {
     return false;
   }
+  const value = attr.value;
+  // Valueless attribute — no `value` property at all.
+  if (!value) {
+    return true;
+  }
   if (value.type === 'GlimmerTextNode') {
-    return value.chars.toLowerCase() === 'true';
+    const chars = value.chars.toLowerCase();
+    // Empty string is exempted (lean toward fewer false positives).
+    return chars === '' || chars === 'true';
   }
   if (value.type === 'GlimmerMustacheStatement' && value.path) {
     if (value.path.type === 'GlimmerBooleanLiteral') {

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

@@ -44,6 +44,10 @@ ruleTester.run('template-no-empty-headings', rule, {
     '<template><h2><@heading /></h2></template>',
     '<template><h3><ns.Heading /></h3></template>',
 
+    // aria-hidden variants — exempt from empty-heading check (fewer-false-
+    // positives policy; see PR body for the four ecosystem positions).
+    '<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>',
     '<template><h1 aria-hidden="TRUE"></h1></template>',
@@ -140,16 +144,10 @@ ruleTester.run('template-no-empty-headings', rule, {
       errors: [{ messageId: 'emptyHeading' }],
     },
 
-    // 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.
+    // Explicit falsy aria-hidden does NOT exempt the empty-heading check —
+    // this is the unambiguous opt-out, no ecosystem position disagrees.
     {
-      code: '<template><h1 aria-hidden></h1></template>',
-      output: null,
-      errors: [{ messageId: 'emptyHeading' }],
-    },
-    {
-      code: '<template><h1 aria-hidden=""></h1></template>',
+      code: '<template><h1 aria-hidden="false"></h1></template>',
       output: null,
       errors: [{ messageId: 'emptyHeading' }],
     },