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From: Philip Kiff
Date: Jun 18, 2025 8:34AM

Hi Claire,

Here are some additional references that support what Paul and Duff say.

When a PDF mixes H headings with H1-6 headings, it means that it is
mixing two different kinds of PDF structuring methods: "Strongly
structured" (using only H's and relying on grouping and nesting to
determine relative values), and "Weakly structured" (using a relatively
flat structure, but relying on the numbering level of the H1-Hn to
determine hierarchy). These structuring methods are defined in PDF
32000-1 in section 14.8.4.3.5 Usage Guidelines for Block-Level Structure.

The PDF/UA-1 specification adds a bit of explicit clarification that
"All documents shall be either strongly or weakly structured, but not
both." at section 7.4.4 in ISO 14289-1.

The PDF/UA-2 specification says that you must use only the "Weakly
structured" method, and so all H's in PDF/UA-2 have to be H1-Hn, as
explained at section 8.2.5.12 Heading (Hn) and (H) in ISO 14289-2.

For WCAG compliance, this would fail under Success Criterion 1.3.1 Info
and Relationships, I think.

Though having said all that, I doubt that there is any real world
accessibility effect on any real person using assistive technology if
the only Heading that uses the unnumbered type of heading is the very
first heading in the document.

Phil.

Philip Kiff
D4K Communications

On 2025-06-18 9:43 a.m., Claire Forbes via WebAIM-Forum wrote:
> Thank you for your reply.
> I guess what I’m trying to understand it why is having one <H> tag considered non-compliant when the rest of the heading tags are <H1> or <H2>, <H3>
>
> From: Paul Rayius <snip>
> Sent: Tuesday, June 17, 2025 4:58 PM
> To: WebAIM Discussion List< <EMAIL REMOVED> >
> Cc: Claire Forbes <snip>
> Subject: RE: <H> tags within PDF
>
> Hi Claire,
> This is a good question; thanks for asking!
> CommonLook PDF throws a failure when a PDF has both and (numbered heading) tags because that is a violation of PDF/UA-1. That said, to address your "other" question, "where does this fail in WCAG": It's important to note that while the WCAG standard does apply very well to PDF, because HTML (web) and PDF are different technologies there is not always a 1:1 mapping between requirements. That said, when it comes to PDF, where there is not a direct alignment between the two specifications, the requirements for PDF have to "win."
> I hope that helps explain CommonLook's approach.
> Best,
> Paul
>
> Paul Rayius, ADS
> VP, Customer Support and Training
>