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Re: Reference/Note tags

for

From: Duff Johnson
Date: Apr 15, 2025 1:06PM

> I didn't see a reply to my response about <Lbl> tags with <Reference> and <Note> tags last week. But the question of how best to tag References is an issue that I notice keeps cropping up in different contexts and forums from time to time, so I thought I'd summarize my own current take on it all.
>
> First, to put this in context, the question of how to use <Lbl> is really a technical standards question that does not currently affect practical accessibility for ANY user agents reading PDF files.

In lists, <LbL> elements distinguish the list item’s label from the <LBody> (the content of the LI). <Lbl> was more broadly (but poorly) defined in PDF 1.x. This situation was addressed in PDF 2.0, but implementations are only now beginning to arrive.

> As far as I know, screen readers, Braille displays, and other assistive technologies don't treat <Lbl> tags any differently than either <Span> or <P> tags,

…when not in the context of a list. We can only hope (and push!) for better implementations.

> and PDF software doesn't actually connect any number or character inside an <Lbl> tag with a corresponding <Reference> or <Note>.

"PDF software" can do whatever it likes, including assisting with the PDF Association’s Best Practice guidance on this point, as you’ve noted… but, yes, the PDF specification doesn’t include special structures for this purpose.

> For that matter, <Reference> and <Note> tags don't really make much difference to anyone either.

Same problem.

> Though <Reference> is sometimes read (incorrectly) as "Link" in some screen readers.

This is a confusing aspect of the old spec, which is why it gets a lot of attention in the Best Practice Guide.

> This will all change when PDF 2.0 becomes more widely adopted, but currently, folks are still targeting PDF 1.7 and PDF/UA-1.

True, and thus, it’s confusing today. :-(

> So at the moment, actual people reading PDFs will not be affected by whatever structure you choose to use for <Lbl> tags. Whew!

Unlike web pages, PDF files persist. Unlike the experience of a browser session, people actually take possession of PDF files, and expect them to work into the (potentially) distant future.

Web pages can be fixed for everyone by a tweak and a reload. By contrast, PDF is often "delivered" rather than "served", so if it’s inaccessible on-delivery it’s generally inaccessible for every downstream user, forever.

That’s why PDF accessibility isn’t - and should never be - just about today’s software.

Ok, I’m a romantic. :-)

> Having said that, I personally think that the "best practice" is to follow the latest version of the Tagged PDF Best Practices Guide from the PDF Association. This is what the original poster, Alan, suggested in the first post in this thread.

+1 :-D

> I have a few additional recommendations for folks aiming to achieve best practices, but these are my own personal suggestions and are not supported by any specific standard or guideline. I like to always add a link to footnote/endnote reference numbers and a backlink from the note to the reference number in the body text so that visual readers can jump back and forth to the footnote and its reference easily.

Software should be able to use the existing structures to do this without links, but in the absence of such I applaud such attention to detail!

> I also recommend enclosing footnote/endnote reference number in square brackets to increase the target size of that link and to generally make it easier for readers to notice that there is a note number there. And I customize the font size of footnote numbers so that they are not so small as to be difficult to read - default note styles using superscript in both Word and InDesign shrink the notes too much and often make it hard for everyone to read.

Nice! Authors, take note!

> Putting all this together, you end up with a tag structure something like (bullets representing nested levels):
>
> <Reference>
> • <Link>
> • • Link-OBJR
> • • [
> • • <Lbl>
> • • • 1
> • • ]
> <Note>
> • [
> • <Lbl>
> • • 1
> • [
> • <P> or <Span> depending on the content of the Note.

+1

> However, as Alan noted, some PDF checking software that will flag one or both uses of <Lbl> above as an error.
>
> Adobe Acrobat's built-in checker flags any <Lbl> that isn't nested inside an <L> as an error by default. You can turn off this check in Acrobat's settings by clearing the "Lbl and LBody must be children of LI" checkbox.
>
> CommonLook PDF I think flags the <Lbl> nested in the <Reference> tag as an error, but passes the <Lbl> nested inside the <Note> tag. I think that's probably based on what I view as an overly restrictive reading of Table 338 - Standard structure types for inline-level structure elements on page 588 of PDF 32000-1, which is referred to in the PDF/UA-1 standard. The Description for the <Note> tag in Table 338 explicitly says that "it may have a label", whereas the description for the <Reference> tag does not mention labels. A more permissive reading like mine allows for a Reference tag also to include a nested <Lbl> tag.

I encourage you to share your observations with the vendors concerned. I know they listen to their users.

> There is a bit more nuance to the question of what the standards actually say because we now know that <Lbl> was incorrectly included in Table 334 - Block-level structure elements on page 585.

Table 334 in PDF 1.7 is… unfortunate.

There’s a reason why the definition of Tagged PDF was overhauled between PDF 1.7 and PDF 2.0!

> And we also know that in the new versions of the PDF standards (PDF 2.0 and ISO 32000-2), <Lbl> is now always considered an inline-level structure - or a special List type. And a new RefNote tag is introduced in PDF 2.0. Though if one wants to future-proof a PDF, using only <Reference> and <Note>, then I think including the <Lbl> as the glue in both is still advisable.
>
> If you want to pass all the checkers, and not have to explain to a client or boss why your PDF fails some checks despite following what you may consider to be best practices, then I would recommend following Laura Robert's very concise and helpful suggestion earlier in this thread to simply not include an <Lbl> tag within a reference tag and use the following structure for the Reference:
> <Reference>
> • <Link>
> • • 1
> • • Link objr

I get it. The lowest-common-denominator usually "wins" while AT users - who, without label tags, are left by themselves to distinguish label from content - are the losers. :-(

We can only hope that, over time, assets such as the PDF Association's Techniques for Accessible PDF will help to elevate common understanding.

Duff.