What is the correct value of HTML 's `returnValue` per spec when the dialog is canceled with Esc?

Check this dialog box implementation.

const open = document.getElementById('open')const dlg = document.getElementById('dlg')open.addEventListener('click', function () {  dlg.showModal()})dlg.addEventListener('close', function () {  console.log('You chose:', dlg.returnValue)})

<button id="open">Open</button><dialog id="dlg"><form method="dialog"><p>Are you sure?</p><button value="no">No</button><button value="yes">Yes</button></form></dialog>

Follow these steps:

1. Run the above code snippet.
2. Click "Open" button.
3. Click "No" in the dialog box.
4. The console log shows You chose: no.
5. Click "Open" button again.
6. Cancel the dialog box by pressing Esc.
7. The console log shows You chose: no.

On Firefox v143, step 7 prints:

You chose: no

On Chrome v140, step 7 prints:

You chose:

Is the value of returnValue when we cancel the dialog box with Esc left undefined in the spec? Or does the spec specify the returnValue in this situation?