[React 19] aria attributes on custom elements behave maybe incorrectly

Summary

In React 18, passing aria-disabled={true} to a custom element serialized that value as "true". That seemed good and as far as I can tell worked as expected.

In React 19, that same code adds the aria-disabled attribute, but doesn't set it to anything. That doesn't work as well. It appears to me (although I'm not certain) that it breaks accessibility for these custom elements. It definitely breaks @testing-library/jest-dom's toBeChecked matcher with aria-checked (https://github.com/testing-library/jest-dom/blob/918b6fbcde10d4409ee8f05c6e4eecbe96a72b7a/src/to-be-checked.js#L31).

For these aria attributes that require strings instead of booleans to function properly, is this an intended change?

Start by comparing the React 18 and React 19 CodeSandbox reproductions for aria-disabled and aria-checked on custom elements. Read the referenced jest-dom src/to-be-checked.js matcher to understand the observed failure, then trace React's custom-element ARIA attribute handling. Done means the intended serialization behavior is established and covered by an appropriate regression test.