EPUB 3.4 Spec in 2026: What Changed and Why It Matters

Q: How do I validate EPUB 3.4 conformance?

Use EPUBCheck 4.4 or later with the --version 3.4 flag. The validator produces a conformance report listing missing fields, accessibility violations, and schema errors.

EPUB 3.4 Spec in 2026: What Changed and Why It Matters

EPUB 3.4 was finalized by the W3C EPUB 3 Community Group in November 2025, and 2026 is the year ebook tools are catching up. If you self-publish, manage a Calibre library, or run a conversion pipeline at work, the new spec introduces changes to metadata, accessibility, and rights expression that will affect your output. This guide walks through what actually changed, which tools support 3.4 today, and what (if anything) you need to update in your workflow.

The good news first: EPUB 3.4 is a backward-compatible incremental release. You don't need to panic. You mostly need to know which three fields are now required for conformance and which one tool to update.

A Brief History of EPUB 3

EPUB 3 has been through four revisions since the format was rewritten for reflowable HTML in 2010. Each release has focused on a different set of priorities — print fidelity, accessibility, metadata hygiene, and (in 3.4) standardization.

Version	Year	Headline change
EPUB 3.0	2010	Reflowable HTML, CSS-based styling
EPUB 3.0.1	2014	Bug fixes and clarifications
EPUB 3.2	2019	Alignment with W3C HTML5/CSS3
EPUB 3.3	2023	Deprecated custom elements, cleaned up overlapping specs
EPUB 3.4	2025	Standardized metadata + accessibility requirements

EPUB 3.4 does not introduce a new file format. An EPUB 3.3 file is still a valid EPUB — the 3.4 spec mainly tightens metadata and accessibility rules for files that want to claim 3.4 conformance.

Headline Changes in 3.4

Five changes matter for anyone producing or managing an ebook library:

dcterms:modified is now required — the modification timestamp metadata field that was optional in 3.3 is mandatory for 3.4 conformance. If you publish to a store that validates against 3.4, your files must include it.
Unified rights expression — 3.4 replaces the loose <meta name="rights"> convention with a formal schema based on the Creative Commons Rights Expression Language (ccREL) and the MARC 21 rights field. The practical effect: store validators can finally parse your license terms programmatically.
WCAG 2.1 AA accessibility is mandatory — 3.3 said "should conform to WCAG"; 3.4 says "must". alt text on images, reading-order metadata, and accessible navigation are now conformance requirements, not best practices.
Refined fixed-layout profile — for textbooks, comics, and design-heavy books, 3.4 tightens the rules around viewport meta tags and CSS pixel dimensions.
The new switch element — a long-requested feature for conditional content. The switch element lets you bundle multiple variants of a passage (e.g. simplified and full versions) and let the reader pick. EPUB 3.4 ships the spec; reader support is still partial.

New Metadata Fields: A Closer Look

The two metadata changes are the ones that will affect the most workflows. Here's a working example of an EPUB 3.4 content.opf file showing both fields:

<metadata xmlns:dc="http://purl.org/dc/elements/1.1/">
  <dc:title>Example Book</dc:title>
  <dc:creator>Jane Author</dc:creator>
  <dc:identifier id="pub-id">urn:uuid:12345</dc:identifier>
  <dc:language>en</dc:language>
  <meta property="dcterms:modified">2026-05-15T10:00:00Z</meta>
  <meta property="rights">CC BY-NC-SA 4.0</meta>
</metadata>

The dcterms:modified value must be an ISO 8601 timestamp in UTC (note the trailing Z). Tools that generate EPUBs from LaTeX, Pandoc, or custom scripts will need to add this field. The rights field accepts a license name or a URL pointing to a full license text.

Inspecting existing files with Calibre:

ebook-meta "my-book.epub"

Calibre 9.9+ displays the dcterms:modified value in the metadata panel and warns if it's missing for 3.4-conformance claims.

Accessibility: WCAG 2.1 AA Is Now Mandatory

This is the change that will have the largest practical impact on publishers. EPUB 3.4 elevates WCAG 2.1 AA from "recommended" to "required" for files that claim 3.4 conformance. Concretely, this means:

All <img> elements must have descriptive alt attributes (decorative images use alt="")
The reading order in the spine must match the visual reading order
Navigation document must list every chapter and section (not just a subset)
Text contrast must meet 4.5:1 for body text and 3:1 for large text
Audio and video must include captions or transcripts

Stores like Apple Books, Google Play Books, and the major library distributors have signalled they will start validating accessibility metadata against EPUB 3.4 in late 2026. Calibre 9.9 ships an accessibility checker that flags the most common issues before you upload.

Calibre 9.9 and EPUB 3.4

Calibre 9.9.0 (released May 28, 2026) is the first version with comprehensive EPUB 3.4 support. Three things changed:

Reads dcterms:modified correctly — 9.8 ignored the field or wrote it in a non-conformant format. 9.9 reads and writes valid ISO 8601 timestamps.
Generates WCAG-compliant output profiles — the new EPUB output profile includes a navigation document that lists all sections, not just chapters, and validates that alt attributes are present on all images.
Exposes the switch element in the editor — you can build conditional content via the built-in EPUB editor, and the output will pass EPUBCheck validation.

Convert any source to EPUB 3.4-compliant output:

ebook-convert "input.docx" "output.epub" \
  --output-profile=tablet \
  --epub-version=3.4 \
  --epub-flatten=False

The --epub-version=3.4 flag is new in 9.9. If you omit it, Calibre defaults to 3.3 (which is still valid EPUB 3, just not 3.4-conformant).

Kobo, Kindle, and EPUBCheck

Three downstream tools you should know about:

Tool	EPUB 3.4 support	Notes
Kobo KEPUB	✅ Native	9.9's native KEPUB output profile preserves `dcterms:modified` and validates against Kobo's reader firmware
Kindle (KFX)	⚠️ Partial	Amazon's KFX format ignores most EPUB 3.4-specific fields; `rights` and `dcterms:modified` are stripped during conversion
EPUBCheck 4.4+	✅ Full	The reference validator now bundles 3.4 schemas and produces conformance reports

Validate an EPUB against 3.4 conformance:

java -jar epubcheck.jar "my-book.epub" --version 3.4

EPUBCheck 4.4 is the first version that knows the 3.4 schema. Older versions will tell you a 3.4 file is invalid because they don't recognize the new fields.

Should I Migrate My Library?

Probably not — but the answer depends on your use case.

If you're a reader: Don't do anything. Your existing EPUBs continue to work. No tool or reader will stop accepting 3.3 or earlier files.

If you're a self-publisher: Update your build pipeline to write dcterms:modified and the new rights field, and run EPUBCheck 4.4 against your output before uploading to stores. Apple Books and Google Play Books will start flagging missing 3.4 fields in late 2026.

If you manage a Calibre library: Update to Calibre 9.9+ to get correct 3.4 reading/writing. You don't need to bulk-resave your existing files — Calibre's on-conversion-write of dcterms:modified happens automatically when you re-save to EPUB.

If you build a conversion SaaS: Update your EPUB output to include both new metadata fields and add an accessibility check pass. The W3C's reference EPUBCheck 4.4 is the easiest way to validate at scale.

FAQ

Q: Do I need to update all my existing EPUBs to EPUB 3.4?
A: No. EPUB 3.4 is backward-compatible — 3.3 and earlier files are still valid EPUB 3. You only need to add the new metadata fields if a store validator requires 3.4 conformance, or if you want the accessibility claims to be valid.

Q: Which Calibre version supports EPUB 3.4?
A: Calibre 9.9.0 (May 2026) is the first version with full support. 9.8 can read 3.4 files but doesn't write the new dcterms:modified field correctly. Update to 9.9+ for both reading and writing.

Q: Will Kindle read EPUB 3.4 files?
A: Only after conversion to KFX or AZW3 via Send to Kindle, Calibre, or a third-party tool. Kindle devices strip the EPUB 3.4-specific fields during conversion, but the file content (text, images, layout) is preserved.

Q: Is EPUB 3.4 the same as EPUB 4?
A: No. EPUB 4 is a separate spec track that the W3C is still drafting. EPUB 3.4 is the final 3.x release.

Q: How do I validate EPUB 3.4 conformance?
A: Use EPUBCheck 4.4 or later with the --version 3.4 flag. The validator produces a conformance report listing missing fields, accessibility violations, and schema errors.

Q: Will EPUBs created before 2026 still work in 2027 and beyond?
A: Yes. The 3.x spec is forward-stable. Files created in 2014 are still valid EPUB 3 today and will continue to be valid in 2030.

Conclusion

EPUB 3.4 is the most consequential minor release of EPUB 3, but it remains a backward-compatible, incremental spec. The two things that will affect most workflows are the mandatory dcterms:modified metadata field and the elevated WCAG 2.1 AA accessibility requirement. Update Calibre to 9.9+, run EPUBCheck 4.4 against your published files, and you're caught up.

For one-off DOCX/HTML/TXT/MD → EPUB conversions where you don't need to manage a full library, converter-epub.com handles the metadata and accessibility boilerplate automatically. The output passes EPUBCheck validation out of the box.

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