README: document what makes the tagging work, and what defeats it

Records that tagging=on is the key doing the work and pdfstandard=ua-2
only writes the claim, since setting the second without the first is
the failure this series fixes and is easy to reintroduce.

Adds the TeX Live 2025 requirement to Dependencies, including that
Debian trixie ships 2024 and that Overleaf projects choose their TeX
Live version in the settings menu and do not always default high
enough.

Adds sections on writing an accessible document (headings, alt text,
table header rows), on customizing the styles, and on verifying that a
PDF really carries structure. The customizing section names titlesec
and titling specifically: they are the obvious tools for the job the
two style files do, and they break tagging silently.

Notes that the poster is not accessible and makes no claim.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README

@@ -50,13 +50,30 @@ All four templates build with LuaLaTeX (via latexmk -lualatex). The text
 fonts are Libertinus Serif and TeX Gyre Heros, both provided by
 texlive-fonts-extra.
 
+The paper, memo, and letter templates need TeX Live 2025 or later. The
+tagging code they rely on is not in TeX Live 2024, where the build
+stops with "the key 'document/metadata/tagging' is unknown". Debian
+trixie ships TeX Live 2024, so a trixie machine needs a newer TeX Live
+from elsewhere in the archive. On Overleaf, choose the TeX Live
+version under Menu > Settings > TeX Live version; 2025 or later is
+required and is not always the default for an older project.
+
 ================================
 === Accessibility ==============
 ================================
 
-All four templates produce PDF/UA-2 compliant tagged PDFs via the
-LaTeX tagging project (\DocumentMetadata + tagpdf). You can verify
-compliance with verapdf: https://verapdf.org/
+The paper, memo, and letter templates produce tagged PDFs that declare
+PDF/UA-2 (ISO 14289-2). The poster template does not; see below.
+
+Tagging is switched on by the tagging=on key in the \DocumentMetadata
+block at the top of each template. That key is what does the work: it
+loads the kernel code that puts headings, paragraphs, lists, tables,
+and figures into the PDF's structure tree, which is what a screen
+reader walks. The neighbouring pdfstandard=ua-2 key only writes the
+conformance claim into the file's metadata. Setting the standard
+without setting tagging=on yields a PDF that announces itself as
+accessible and contains nothing for assistive software to read, so
+keep the two together.
 
 The paper and memo templates were originally built on the memoir
 class, which is incompatible with the LaTeX tagged-PDF code. They now
@@ -64,6 +81,67 @@ use the standard article class with custom styling (cdsc-paper.sty
 and cdsc-memo.sty) to keep their previous look while supporting full
 tagging.
 
+Writing an accessible document
+------------------------------
+
+Tagging gets you the structure; the content is still up to the author.
+Three things matter most, and the paper template carries a worked
+example of each:
+
+  * Use \section and \subsection for headings rather than setting type
+    in bold by hand. Screen reader users navigate by the heading tree,
+    and hand-formatted text is invisible to it.
+
+  * Give every figure an alt= description on \includegraphics, saying
+    what the reader is meant to take from it. A decorative image takes
+    \includegraphics[artifact]{...} instead, keeping it out of the
+    reading order.
+
+  * Mark header rows on data tables with \tagpdfsetup{table/header-rows
+    ={1}} so their cells become TH rather than TD. Without it a table
+    reads as an undifferentiated grid of values.
+
+Customizing the styles
+----------------------
+
+Some common packages defeat tagging, and the failure is usually
+silent: the document compiles without complaint and the tags are
+simply missing. Before adding a package, check its status against the
+LaTeX Tagging Project's list:
+
+  https://latex3.github.io/tagging-project/tagging-status/
+
+titlesec and titling are the two to know about here, because they are
+the obvious tools for the job cdsc-paper.sty and cdsc-memo.sty do.
+titlesec replaces the \@startsection hook that the tagging code uses
+to emit heading tags, so loading it produces a PDF with no headings in
+the structure tree and no error to say so. Both style files therefore
+build their headings and title blocks from the article class's own
+\@startsection and \@maketitle hooks.
+
+Verifying
+---------
+
+To see whether a PDF really carries structure, open the tag tree in a
+PDF reader that shows one, or check for the tags directly:
+
+  python3 -c "import pikepdf,sys; d=pikepdf.open(sys.argv[1]); \
+    print(d.Root.get('/StructTreeRoot') and 'tagged' or 'UNTAGGED')" file.pdf
+
+That confirms structure exists but not that the file conforms to
+PDF/UA-2. Full conformance checking needs veraPDF
+(https://verapdf.org/), which is not packaged in Debian.
+
+Posters
+-------
+
+The poster template is not accessible and makes no accessibility
+claim. The beamer class rejects \DocumentMetadata outright, and the
+LaTeX Tagging Project lists beamer and beamerposter as no-support,
+meaning incompatible and not expected to change. When a poster needs
+an accessible version, build a companion document from the paper or
+memo template.
+
 ================================
 === Branches ===================
 ================================