Make PDF documents conform to PDF/UA-1 standard

[u451f]

Identified for now:

• [x] metadata language in colophon 7f3f29137da16c12bd495f8b304e4453b6cd8599
• [x] generate documents with PDF/UA-1 option 8999e52f991f0ed0c417316566e73f21e0be358c
• [ ] paged-js-cli may offer a similar PDF/UA-1 option → to research or switch to using Weasyprint for the front and backcover generation (we did not, because the graphic output was flawed)
• [ ] check if our combination of the different PDF parts results in a compatible document
• [ ] verify more issues on https://demo.verapdf.org or locally (installing veraPDF) →Check errors against ISO standard (but buying access to the standard costs 216CHF)
• [x] find a CLI validator for ease of use → There's veraPDF and there's a RFP (request for package) in Debian

[u451f]

Testing only the contents of CIL006: Specification: ISO 14289-1:2014, Clause: 7.18.5, Test number: 1

Links shall be tagged according to ISO 32000-1:2008, 14.8.4.4.2, Link Element   Failed
22 occurrences
PDLinkAnnot 
structParentStandardType == 'Link' || isOutsideCropBox == true || (F & 2) == 2  
root/document[0]/pages[0](6 0 obj PDPage)/annots[0](7 0 obj PDLinkAnnot)

→ I suspect that is related to the links in the table of contents. Likely we have not much influence on these, and we need to open a bug report for Weasyprint, once we confirm my hunch is indeed correct. → This is confirmed. It's related to the generation of the ToC.

[u451f]

Another report was that the language tag was missing, which is fixed by adding the lang element to the metadata, as done in 87763dc. This needs to be documented.

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Helpful document: https://pdfa.org/wp-content/uploads/2019/06/TaggedPDFBestPracticeGuideSyntax.pdf

[u451f]

Another error: Specification: ISO 14289-1:2014, Clause: 7.2, Test number: 19

I did not (yet) find out which are these elements, i probably lack the right tool to look into it more in detail.
L element may contain only L, LI and Caption elements   Failed
9 occurrences

Totally unclear what this means. Check with pandoc -s -t native CIL006/3main.md

[u451f]

Specification: ISO 14289-1:2014, Clause: 7.4.2, Test number: 1

For documents that are not strongly structured, as described in ISO 32000-1:2008, 14.8.4.3.5, heading tags shall be used as follows: (*) If any heading tags are used, H1 shall be the first. (*) A document may use more than one instance of any specific tag level. For example, a tag level may be repeated if document content requires it. (*) If document semantics require a descending sequence of headers, such a sequence shall proceed in strict numerical order and shall not skip an intervening heading level. (*) A document may increment its heading sequence without restarting at H1 if document semantics require it   Failed
3 occurrences

→ This could be due to the fact that we use H6 for pulled quotes. But needs to be confirmed