incidentally... twbs/bootstrap#37210

Yes, but it is a failure of the HTML specification.

There are 2 statements:

• SHOULD: if you have headings, you should start with h1
• MUST: if you have different heading levels, you must use correct levels without skipping one level (after h1 no h3, h4, h5 or h6 is allowed, only h2)

Skipping levels is not a failure of the HTML specification (or at least, not the one linked to). Emphasis added:

Each heading following another heading lead in the outline must have a heading level that is less than, equal to, or 1 greater than lead's heading level.

That is not the same as @JAWS-test paraphrase:

If you have different heading levels, you must use correct levels without skipping one level

After h1 — h3, h4, h5 or h6 is allowed because h3-h6 are less than h1.

AAA SC 2.4.10 Section Headings can be used to fail a page that skips heading levels, but not 1.3.1.

2.4.6 is Headings and describe topic or purpose. I agree with you that headings (1) are not required (at AA), and (if present) might not actually be all that descriptive and (2) still meet 2.4.6.

I wrote:

AAA SC 2.4.10 can be used to fail a page that skips heading levels, but not 1.3.1.

I wish I had written "might" or "may" and not "can" as I think it is possible but not necessarily the case that skipping levels fails 2.4.10.

I think we agree than an auditor should fail blatant misuse of headings under 1.3.1 (e.g. using CCS to flip the visual prominence of H2 and H3, while organizing content based on the visual prominence). But if the author's choices for H2/H3 (both visual and programmatic) are not consistent, that probably fails 2.4.10 but not 1.3.1.

@bruce-usab I'm not sure what you mean for the latter but we can chat about it some other time... off topic for this issue

I interpreted that bit as snark.

From: 4.3.11 Headings and outlines

The lower a heading's heading level is, the fewer ancestor sections the heading has.

H1 has no ancestors. H2 has at least one ancestor. H1 is a lower heading level than H2. But the line in question, does not use lower it uses less than.

From a hierarchal view, H4 < H2

Each heading following another heading lead in the outline must have a heading level that is less than, equal to, or 1 greater than lead's heading level.

H4 can follow H2 because H4 < H2

H4 can follow H4 because H4 is equal to H4

What I am not quite groking is that, apparently, only H3 can follow H4? (1 greater)

So an outline of: H2, H4, H4, H2 — is not valid HTML?

But I think I must be missing what is meant by heading lead because it would have been much more straightforward for the requirement to be written as:

Each heading following another heading in the outline must be a lower heading level, the same level, or one greater heading level.

It does not say that, so I infer that the concept of heading lead must be integral to the meaning.

Here is a version using lower than instead of less than:

Each heading following another heading in the outline must have a heading level that is lower than, equal to, or 1 greater than the previous heading level.

I interpreted that bit as snark.

why would it be snark? it reflects exactly what the spec says https://html.spec.whatwg.org/multipage/sections.html#outline

h1–h6 elements have a heading level, which is given by the number in the element's name.

heading level is given by the number

Each heading following another heading lead in the outline must have a heading level that is less than, equal to, or 1 greater than lead's heading level.

it then tells you the heading level of the next heading must be less than, equal, or 1 greater than that of the heading. as heading level is the number, the number must be <, =, or +1

don't think there's any different way to read this?

and regarding the confusion there on what "heading lead" means, it is not a term that seems defined anywhere, but I'm reading this as a way of identifying which heading you're then talking about. rewritten differently

Each heading following another heading X in the outline must have a heading level that is less than, equal to, or 1 greater than heading X's level.

and again less than, equal to, greater than, lower than, higher than, all these terms refer to the number, not any kind of hierarchical semantic meaning. so h1 is lower than h2, h1's heading level is less than h2's heading level.

read domenic's statement again, not as snark, but as a statement of fact

Well, note that it's not comparing the headings; it's comparing the heading levels. And the heading levels are pretty clearly defined as numbers. Numbers have known semantics for "greater" and "less".

and domenic confirms that i'm correct in my understanding whatwg/html#8366 (comment)