diff --git a/README b/README
index 90f12ea..1c74874 100644
--- a/README
+++ b/README
@@ -36,10 +36,47 @@ On Debian or Ubuntu, install the following packages:
 On other systems, installing a full TeX Live distribution should
 provide everything needed.
 
+TeX Live 2025 or later is required. The tagging code is not in TeX
+Live 2024, where the build stops with "the key
+'document/metadata/tagging' is unknown". Debian trixie ships TeX Live
+2024. On Overleaf, set the version under Menu > Settings > TeX Live
+version; it is not always high enough by default on an older project.
+
 The two required fonts Matrix-II and OpenSans are both included in
 this repository, so no additional font packages are needed.
 
 
+Accessibility
+=============
+
+The letter is a tagged PDF declaring PDF/UA-2 (ISO 14289-2). Two keys
+in the \DocumentMetadata block at the top of the .tex file do this,
+and both are needed: tagging=on loads the kernel code that puts the
+text into the PDF's structure tree, and pdfstandard=ua-2 writes the
+conformance claim. The claim without the tagging produces a letter
+that passes a conformance checker while containing nothing a screen
+reader can read, so keep them together.
+
+Check a built letter with veraPDF (https://verapdf.org/, not packaged
+in Debian):
+
+  verapdf -f ua2 washington_letterhead_letter-matrix-deptartment.pdf
+
+Note that a PASS is necessary but not sufficient. A letter whose body
+text is all marked as artifact passes too, because the standard only
+requires that content which is not real be an artifact. Confirm there
+is something in the structure tree as well:
+
+  python3 -c "import pikepdf,sys; d=pikepdf.open(sys.argv[1]); \
+    print(d.Root.get('/StructTreeRoot') and 'tagged' or 'UNTAGGED')" file.pdf
+
+The signature image carries an alt= description, which is what a
+screen reader announces in place of it. Keep that in place if you
+swap in your own signature. The letterhead graphic in the page header
+is treated as an artifact, like other running header content, so it
+is skipped rather than described.
+
+
 Encrypted signature files
 ===========================
 
diff --git a/washington_letterhead_letter-matrix-deptartment.tex b/washington_letterhead_letter-matrix-deptartment.tex
index 5d35bee..c57002a 100644
--- a/washington_letterhead_letter-matrix-deptartment.tex
+++ b/washington_letterhead_letter-matrix-deptartment.tex
@@ -1,4 +1,15 @@
+% Tagged PDF (PDF/UA-2) accessibility setup.
+%
+% tagging=on is what actually builds the structure tree: it loads the
+% kernel code that tags paragraphs, headings, and figures. Declaring
+% pdfstandard=ua-2 on its own only writes the claim into the metadata.
+% Without tagging=on this letter validated as PDF/UA-2 while its
+% structure tree held nothing but /Document, its body text was marked
+% as artifact, and the alt= descriptions below were silently dropped:
+% a file that passes the checker and is unreadable to a screen reader.
+% Needs TeX Live 2025 or later.
 \DocumentMetadata{
+  tagging=on,
   pdfversion=2.0,
   pdfstandard=ua-2,
   lang=en-US,
@@ -17,8 +28,6 @@
 \usepackage[top=1.25in,left=1.25in,bottom=1.20in,right=1.25in]{geometry}
 
 \usepackage{fontspec}
-\usepackage{tagpdf}
-\tagpdfsetup{activate-all}
 \newopentypefeature{Contextuals}{NoAlternate}{-calt}
 \defaultfontfeatures{Kerning=Uppercase,Ligatures=TeX}