Test 4 on this test page fails
https://thepaciellogroup.github.io/AT-browser-tests/test-files/output.html

expected behaviour: the child text of the label element is the accessible name for the output element

observed behaviour: output element has no accessible name

code example:

<label>
	Label wrapping output
	<output id="output4">10</output>
</label>

I think this is due to this check which excludes labelling by ancestor for elements which aren't "widgets":
https://searchfox.org/mozilla-central/rev/0e309417bdc73be33fc93f6bd7a0d4063d2c6e51/accessible/base/AccIterator.cpp#133
That was added in bug 687393. However, in bug 1226000, an IsLabel method was added which we might be able to use instead of checking for widget.

Sorry, this is a false issue, the output element only requires an acc name from aria-label and aria-labelledby https://w3c.github.io/html-aam/#output-element-accessible-description-computation

We might have a bug in the HTML AAM here, too. I haven't checked the actual HTML spec yet, but MDN says that the output element is "labelable":
https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Content_categories#Form_labelable
Mozilla's DOM code also agrees (despite the a11y engine's failure to honour this). Unless both MDN and Mozilla DOM are wrong?

I guess maybe this was done in HTML AAM so the subtree is used in preference to a wrapping label? Hmm. I think I'm just going to leave this alone.