I think that for the most part I like this proposal. I have three 'concerns'.. The first two are mostly about impact and messaging.
The first is just that as steve said earlier it'd be effectively building in a pattern that would create something we historically advise authors to explicitly not to write manually. That could use a solid explanation of why everyone is ok flipping on that (and confirmation that they are) or I think that could go badly.
Second is that as it stands, for those people who spent a lot of time and effort using sectioning and made good headings that didn't rely on the phantom 'document outline' that never shipped, the beautiful AT heading relationships, the conceptual 'outline' that they built will get less beautiful. For example, given something like:
<article>
<h1>Apple varieties</h1>
<p>The apple is the pomaceous fruit of the apple tree...</p>
<section>
<h2>Red Delicious</h2>
<p>These bright red apples are the most common found in many supermarkets...</p>
</section>
<section>
<h2>Granny Smith</h2>
<p>These juicy, green apples make a great filling for apple pies...</p>
<div role="heading" class="note" aria-level="3">The origin of the name</div>
<p>...blah...</p>
</section>
</article>
Today, this has a pretty decent AT tree and comminicates sensible heading relationships. While there is no 'outline', it's totally all 'there' for someone to make sense of with the navigational tools available in the AT... If you used them (but I'll come back to that). Some even use this their to their advantage to autogenerate TOCs and things so that everyone benefits from this, and the relationships look like this:
- Apple Varieties
- Red Delicious
- Granny Smith
- The origin of the name
But, the result of this approach to their AT trees is that it gets less sensible by the measures we have previously laid out. In other words, it's been stressed over the years "it's quite important you get this right" and "here's the critieria for what it means to be 'right'". Some people, maybe not that many, but some, for whatever reason and sometimes at great cost (sometimes by force of law) went back and tried really hard to meet those criteria, and now by those same measures it seems like we are undoing some of that. That's really the thing that worries me just a bit, because it feels like it could kind of be... disheartening.
I want to stress that I don't actually know how important this is, I'm just pointint it out as a thing to consider. It definitely should have some strong messaging about why I think if that's the route. I've said before that I think that numbers are less important than relationships and that that is confusing right now because the only relationships anyone has to make sense of the world is via numbers that implied structure and not the other way round. My experience is limited, but from the folks I've talked to, depite what we encourage, I don't think people 'use' the tools in readers to think about structures that much precisely because they have always been so broken 99% of the time and they were based on really 'flat' models. Maybe the best thing to do is to just admit that and provide a good plan going forward, and this might be it.
My third thing is that none of the proposed solutions (even mine so far) that seem 'doable' have dealt well IMO with the reality that Jake mentions and I think that's potential fatal to real success. Very often a site (or sections of a site) can be very structured, but what is frequently just referred to as content is authored in a way a lot more like it is read - either with a decent rich text editor or as simple markdown or something. Speaking "by volume" this is a huge huge huge amount of the web for the simple reason that tools like this dramatically increase the potential number of authors by specifically being not intensely structured, more like the traditional 'flat' documents. It's entirely possible to author "good" content with good/sensible headings that isn't really 'compatible' with inclusion in your pretty structured page.
So, I have what I think is a rather small tweak to suggest: What if we added to this proposal a non-reflected, non-dynamic policy attribute or something (forgive my potential butchering/overloading of terms here as I try to explain) so that authors could simply identify upfront that this particular container will hold 'flat' content so that we could maybe choose a slightly different strategy there. In these, we'd simply use Anne's logic to calculate the effective level of the container and then add that to the specified tag heading level. Thus you could have something like
<section>
<h1>One</h1>
<section>
<h1>Two</h1>
<article heading-policy="flat">
<!-- this is simply included 'flat' content -->
<h1>Apple varieties</h1>
<p>The apple is the pomaceous fruit of the apple tree...</p>
<h2>Red Delicious</h2>
<p>These bright red apples are the most common found in many supermarkets...</p>
<h2>Granny Smith</h2>
<p>These juicy, green apples make a great filling for apple pies...</p>
</article>
</section>
</section>
</section>
and the result would be like
<section>
<h1 aria-level="2">One</h1>
<section>
<h1 aria-level="3">Two</h1>
<article heading-policy="flat">
<h1 role="heading" aria-level="4">Apple varieties</h1>
<p>The apple is the pomaceous fruit of the apple tree...</p>
<h2 role="heading" aria-level="5">Red Delicious</h2>
<p>These bright red apples are the most common found in many supermarkets...</p>
<h2 role="heading" aria-level="5">Granny Smith</h2>
<p>These juicy, green apples make a great filling for apple pies...</p>
</article>
</section>
</section>
I have hacked a mod of Anne's polyfill which does this, it seems like a fairly minor tweak? Again, I'm suggesting this would only be at parse time and not live, that is, adding or removing the attribute (or however it might be done) wouldn't affect things. This is probably easier to think about if it was a tag (since that would better explain the fact that it has to be there and is unchangable) but I think it's harder to separate them a little. Maybe it would make sense to float a custom element and see if people used it/correctly?