This PR is part of a broader a11y parity audit comparing our rules against jsx-a11y, vue-a11y, angular-eslint-template, and lit-a11y.

Premise

Per aria-query 5.3.2, a handful of ARIA attributes accept the literal string "undefined" as a spec-valid value:

Two orthogonal encodings: token with 'undefined' in values (aria-orientation) and boolean with allowundefined: true (the other four). Both should produce "accept the string 'undefined'"; neither is the user's fault to write.

Problem

The pre-fix isValidAriaValue had a clunky two-layer structure:

if (value === 'undefined' && isTokenWithUndefinedInValues) return true;   // precheck
if (value === 'undefined') return Boolean(attrDef.allowundefined);        // short-circuit
return validateByType(...);

The precheck was a late patch that landed because aria-orientation="undefined" (the token case) was falsely rejected by the short-circuit. The short-circuit itself exists for the 4 boolean-type attrs. So validation was split across three locations for one attribute.

Fix

Absorb allowundefined handling directly into validateByType via a single helper allowsUndefinedLiteral(attrDef, value). The token-values check in the 'token' case naturally handles aria-orientation="undefined" without any special-casing. isValidAriaValue becomes a thin dispatcher to validateByType.

function allowsUndefinedLiteral(attrDef, value) {
  return value === 'undefined' && Boolean(attrDef.allowundefined);
}

function validateByType(attrDef, value) {
  if (allowsUndefinedLiteral(attrDef, value)) return true;
  switch (attrDef.type) {
    // 'token' case accepts 'undefined' naturally via the values list for aria-orientation
    // 'boolean' case rejects string 'undefined' (the allowundefined branch above handles the allowed cases)
    // ...
  }
}

Behavior — unchanged outcomes, cleaner structure

Prior art — peers don't handle the 4 boolean+allowundefined attrs correctly

Verified each peer in source:

So of the four peers, three reject a spec-valid construct, one doesn't check values at all. This PR makes our rule more spec-compliant than the three value-validating peers on these 4 attrs.

Note — case-insensitivity gap (out of scope)

jsx-a11y and lit-a11y lowercase the value before the token check (permittedValues.indexOf(value.toLowerCase()) > -1). Our rule does NOT lowercase; a case-variant like aria-orientation="UNDEFINED" or "Horizontal" would still fail here even after this PR. Same class of HTML-enumerated-attribute case-insensitivity gap as #2718 (kind="captions") and #2728 (role tokens). Worth a follow-up PR; explicitly out of scope for this narrow change.

Tests

• Valid: aria-orientation="undefined" / "horizontal", aria-expanded="undefined", aria-hidden="undefined", aria-grabbed="undefined", aria-selected="undefined", aria-pressed="true" / "false" / "mixed".
• Invalid: aria-orientation="sideways", aria-pressed="undefined" (no allowundefined), aria-required="undefined" (boolean without allowundefined — pre-existing test).