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Re: Reference/Note tags

for

From: Philip Kiff
Date: Apr 14, 2025 11:18AM

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

For that matter, <Reference> and <Note> tags don't really make much
difference to anyone either. Though <Reference> is sometimes read
(incorrectly) as "Link" in some screen readers. 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.

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

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.

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. 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.

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.

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.

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. 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'd still recommend surrounding the note number in square brackets, but
that won't affect whether you pass a validator.

I'd welcome any corrections or alternate views on any of this!

Phil.

Philip Kiff
D4K Communications

On 2025-04-10 2:57 p.m., Philip Kiff via WebAIM-Forum wrote:
> Following up on the use of Lbl (Label) tags in PDF/UA-1. I guess I'm
> not sure where in the various standards it says that we can't have an
> Lbl tag nested inside a Reference tag?
> [....]
>
> On 2025-04-10 10:08 a.m., Paul Rayius via WebAIM-Forum wrote:
>> [....] So, while it might be allowed, and appropriate, in the PDF 2.0
>> / PDF/UA-2 context, to have a Lbl inside a Reference, that's not
>> allowed in PDF/UA-1.