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diff --git a/README.md b/README.md
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--- a/README.md
+++ b/README.md
@@ -59,6 +59,19 @@ zig build test           # run unit tests
 
 EPUBs land in `epubs/zig-<version>.epub`.
 
+### Validating output
+
+The tool guarantees well-formed XHTML, but full EPUB3 conformance (the kind of
+thing Kindle's "Send to Kindle" enforces) is checked with
+[epubcheck](https://github.com/w3c/epubcheck), the official validator:
+
+```sh
+brew install epubcheck   # one-time (needs a JRE)
+./validate.sh            # runs epubcheck on every epub; non-zero exit on errors
+```
+
+`validate.sh` fails loudly if any EPUB has errors, so it doubles as a CI gate.
+
 ## Continuous integration
 
 `.forgejo/workflows/build-epubs.yml` runs on [Codeberg](https://codeberg.org)