[tabs] Concern about heading declarations as boundaries

I'm coming in from https://daverupert.com/2021/10/native-html-tabs/ and noticed a couple of things that might lead to some delimiting confusion when not using delimiters/containers around the sections. I couldn't really find a central design doc around these after quite a few minutes of grepping around in search bars and poking around in directories.

1. role=heading aria-level=7 is a way to have more than <h6> level headings but in theory it could also be applied to elements. I'm guessing these would be picked up? In particular mixing <h1> and aria-level=1 would both have them be sections?

2. mixing headings is quite possible so tracking the "top level" heading value seems like it needs to occur but it can be confusing in poor markup scenarios like:

   1. having a heading level that appears to be out of order

<spicy-section>
  <h2>post A</h2>
  hi
  <h1>port B</h1>
  there
</spicy-section>

2. having a nested heading in a section

<spicy-section>
  <h1>movie 1</h1>
  <h2>cast</h1>
  Bob
  <h1>movie 2</h1>
  there
</spicy-section>

3. normally in screen readers tab titles and accordion summaries aren't treated as part of the heading navigation, I guess this would enforce that they are if they are using heading semantics to split up the interface. It seems like being able to have those without the header role would be nice since the common case today is not for them to be treated as headers in my own experience and looking in WCAG it doesn't look like they are normally.