https://www.w3.org/WAI/ARIA/wiki/ARIA_1.1_Changes

I would push back on the spec on this. It doesn't make sense, because the AT will no longer be able to tell the difference between a fake level 2 and a real 2. By leaving the value undefined, the AT can decide the best behavior.

This is really a matter of separating the model from the view. The model should be exposed as intact as possible.

It would be like having domElement.getAttribute('foo') return a default value when the attribute was not there.

I remember that there were other aria attributes that returned default values other than Undefined when not present. I can't think of anything off the top of my head, but perhaps aria-invalid?
That's an area of the Spec that confused me as well.

Aaron, assigning to you to raise the issue with Mozilla, the ARIA group, etc. and try to reach consensus on this.

Dear @Aaron,
Does it start to investigate on W3C's public ARIA?
If yes, could you share the progress?

Dear @dmazzoni @aleventhal
I'm not sure that is still in progress or not.
Is it OK if I upload a patch for current issue?

I just spoke with Aaron about this and he still feels like this is not something Chrome should do, but rather, it should be left up to the AT to decide. I'm going to remove the GoodFirstBug label, and if anyone is interested maybe they could follow up with public-aria@w3.org and get some feedback.

Upon further investigation, it turns out Chromium is already returning a heading level of 2 if an element has role=heading but no aria-level.

https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/accessibility/AXNodeObject.cpp?l=1141

Fixed here: https://chromium-review.googlesource.com/c/chromium/src/+/670539

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/aff98621c9dff2219a85859fa123744d8749c8ea

commit aff98621c9dff2219a85859fa123744d8749c8ea
Author: Rob Dodson <robdodson@chromium.org>
Date: Tue Jan 09 01:13:38 2018

Test elements with heading role but no aria-level return a level of 2.

Fills in a missing LayoutTest that checks that any element with role="heading"
but no specified aria-level will return a level of 2 by default.

R=aboxhall@chromium.org

Bug:  688580 
Change-Id: I9835bfab0c40e97fc9e2d503ededeed5b225b057
Reviewed-on: https://chromium-review.googlesource.com/849659
Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Commit-Queue: Rob Dodson <robdodson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#527845}
[modify] https://crrev.com/aff98621c9dff2219a85859fa123744d8749c8ea/third_party/WebKit/LayoutTests/accessibility/heading-level-expected.txt
[modify] https://crrev.com/aff98621c9dff2219a85859fa123744d8749c8ea/third_party/WebKit/LayoutTests/accessibility/heading-level.html