fix(template-anchor-has-content): align aria-hidden detection with WAI-ARIA spec

Per WAI-ARIA 1.2 §6.6 + aria-hidden value table, a missing or empty-string
aria-hidden resolves to the default `undefined` — NOT `true`. So
<span aria-hidden>X</span> as a child of <a href="/x"> does NOT hide the
span; its content still contributes to the anchor's accessible name.

The prior behavior inherited jsx-a11y's JSX-coercion convention and
vue-a11y's "anything-not-literal-false" shortcut. Both are peer-plugin
conventions that diverge from normative ARIA. Matches the spec-first
resolution of ember-cli#2717, #19, and #33.

Moved valueless / empty aria-hidden cases from invalid → valid. Kept the
explicit aria-hidden="true" and {{true}} cases as invalid.

Author: johanrd

lib/rules/template-anchor-has-content.js

@@ -20,25 +20,18 @@ function isDynamicValue(value) {
   return value?.type === 'GlimmerMustacheStatement' || value?.type === 'GlimmerConcatStatement';
 }
 
-// Returns true if the `aria-hidden` attribute is effectively truthy. Mirrors
-// the jsx-a11y/vue-a11y convention: valueless (`aria-hidden`), string `"true"`,
-// or `{{true}}` all hide the element from the a11y tree. Dynamic values are
-// treated as "hidden" only when the developer explicitly passes boolean true;
-// anything we cannot statically resolve falls through as not-hidden so we
-// don't silently swallow meaningful content.
+// Returns true if the `aria-hidden` attribute is explicitly set to "true"
+// (case-insensitive) or mustache-literal `{{true}}`. Per WAI-ARIA 1.2 §6.6
+// + aria-hidden value table, valueless / empty-string `aria-hidden` resolves
+// to the default `undefined` — NOT `true` — so those forms do NOT hide the
+// element per spec. This aligns with the spec-first decisions in #2717 /
+// #19 / #33, and diverges from jsx-a11y's JSX-coercion convention.
 function isAriaHiddenTrue(attr) {
-  if (!attr) {
+  if (!attr?.value) {
     return false;
   }
-  // Valueless attribute (e.g. `<span aria-hidden />`) parses with no value.
-  if (attr.value === undefined || attr.value === null) {
-    return true;
-  }
   if (attr.value.type === 'GlimmerTextNode') {
-    const chars = attr.value.chars.trim().toLowerCase();
-    // HTML parses bare `aria-hidden` as `aria-hidden=""`; treat empty as true
-    // to mirror the valueless shape above.
-    return chars === '' || chars === 'true';
+    return attr.value.chars.trim().toLowerCase() === 'true';
   }
   if (attr.value.type === 'GlimmerMustacheStatement') {
     const path = attr.value.path;

tests/lib/rules/template-anchor-has-content.js

@@ -64,6 +64,21 @@ ruleTester.run('template-anchor-has-content', rule, {
       filename: 'test.gjs',
       code: '<template><a href="/x"><span aria-label="close icon" /></a></template>',
     },
+
+    // Valueless / empty aria-hidden resolves to default `undefined` per
+    // WAI-ARIA 1.2 §6.6 — the child is NOT hidden, its content counts.
+    {
+      filename: 'test.gjs',
+      code: '<template><a href="/x"><span aria-hidden>X</span></a></template>',
+    },
+    {
+      filename: 'test.gjs',
+      code: '<template><a href="/x"><span aria-hidden="">X</span></a></template>',
+    },
+    {
+      filename: 'test.gjs',
+      code: '<template><a href="/x"><img aria-hidden alt="Nope" /></a></template>',
+    },
   ],
 
   invalid: [
@@ -88,23 +103,17 @@ ruleTester.run('template-anchor-has-content', rule, {
       output: null,
       errors: [{ messageId: 'anchorHasContent' }],
     },
-    // aria-hidden subtree contributes nothing to the accessible name.
+    // aria-hidden="true" subtree contributes nothing to the accessible name.
     {
       filename: 'test.gjs',
       code: '<template><a href="/x"><span aria-hidden="true">X</span></a></template>',
       output: null,
       errors: [{ messageId: 'anchorHasContent' }],
     },
+    // <img aria-hidden="true" alt="Nope" /> — alt not exposed when hidden.
     {
       filename: 'test.gjs',
-      code: '<template><a href="/x"><span aria-hidden>X</span></a></template>',
-      output: null,
-      errors: [{ messageId: 'anchorHasContent' }],
-    },
-    // <img aria-hidden /> — alt is not exposed when the image is hidden.
-    {
-      filename: 'test.gjs',
-      code: '<template><a href="/x"><img aria-hidden alt="Nope" /></a></template>',
+      code: '<template><a href="/x"><img aria-hidden="true" alt="Nope" /></a></template>',
       output: null,
       errors: [{ messageId: 'anchorHasContent' }],
     },
@@ -170,6 +179,10 @@ hbsRuleTester.run('template-anchor-has-content (hbs)', rule, {
     // Anchors without href are out of scope.
     '<a />',
     '<a>Foo</a>',
+    // Valueless aria-hidden resolves to default `undefined` per ARIA §6.6 —
+    // child is not hidden, its content counts.
+    '<a href="/x"><span aria-hidden>X</span></a>',
+    '<a href="/x"><img aria-hidden alt="Nope" /></a>',
   ],
   invalid: [
     {
@@ -183,12 +196,12 @@ hbsRuleTester.run('template-anchor-has-content (hbs)', rule, {
       errors: [{ messageId: 'anchorHasContent' }],
     },
     {
-      code: '<a href="/x"><span aria-hidden>X</span></a>',
+      code: '<a href="/x"><span aria-hidden="true">X</span></a>',
       output: null,
       errors: [{ messageId: 'anchorHasContent' }],
     },
     {
-      code: '<a href="/x"><img aria-hidden alt="Nope" /></a>',
+      code: '<a href="/x"><img aria-hidden="true" alt="Nope" /></a>',
       output: null,
       errors: [{ messageId: 'anchorHasContent' }],
     },