diff --git a/README b/README
index 02b7666..eca5413 100644
--- a/README
+++ b/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.
+
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 === Branches ===================
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