What EPUB Validation Actually Does
Before your ebook lands on Apple Books, Amazon KDP, or any major retailer, it goes through validation—a technical scan that checks whether your EPUB file meets the EPUB 3 standard. Think of it as a building inspector before you move in: it catches structural problems that readers will experience as broken formatting, missing images, or navigation that doesn't work.
The frustrating part? Validation errors are often invisible in your Word manuscript. You might have perfectly formatted chapters, clean chapter breaks, and embedded images that look great in Word. But once converted to EPUB, validation tools flag issues that prevent distribution or cause retailers to reject your file outright.
Understanding these failures—and how to prevent them—saves you weeks of back-and-forth with retailers and multiple re-uploads.
The 7 Most Common EPUB Validation Failures
1. Broken Internal Links and Navigation
Your table of contents won't work, or links to chapters point to the wrong sections. This happens when:
- Chapter headings aren't styled consistently (some as Heading 1, others as bold text)
- You've manually typed chapter references instead of using cross-references
- Heading IDs don't match the anchor points they're supposed to link to
The fix: In Word, apply Heading 1 style to all chapter titles before conversion. Use Insert → Link → Bookmark to create internal cross-references, not manual text. Validation tools will then correctly map your TOC to actual chapter locations.
2. Missing or Incorrectly Named Image Files
You embedded images in Word, but the EPUB validator can't find them or they're stored with broken file paths. Common causes:
- Images linked externally rather than embedded
- Unsupported image formats (BMP, TIFF, or other non-standard formats in an EPUB)
- File names with spaces or special characters that break path references
The fix: Embed all images directly in Word (Insert → Pictures → This Device). Use JPG or PNG only, and rename files to remove spaces (use hyphens or underscores instead). EPUB 3 requires valid, embedded image references; external links won't work in most readers.
3. Incorrect Language Declaration
Your EPUB metadata doesn't specify the language, or it's set incorrectly. Retailers and accessibility tools rely on this to render text properly (especially for hyphenation and screen readers).
The fix: Before conversion, check your Word document's language setting (File → Options → Language). Set it to the correct language (English US, English UK, Spanish, etc.). When you upload to ebookconvert.pro, the Book Details form explicitly asks for language—select the right one there too.
4. Orphaned or Malformed Styles
You've applied custom styles in Word that don't translate cleanly to EPUB. The converter tries to preserve them, but they end up as inline CSS that validators flag as redundant or broken.
The fix: Before conversion, strip custom styles and use only Word's built-in styles (Heading 1, Heading 2, Normal, Emphasis, Strong, etc.). If you've created a custom "ChapterTitle" style, replace it with Heading 1. The cleaner your Word styles, the cleaner your EPUB output and the fewer validation errors you'll face.
5. Duplicate or Missing Metadata Fields
Required metadata (title, author, language) is either blank, duplicated, or formatted incorrectly. Retailers won't accept an EPUB without complete metadata, and validation tools will flag gaps.
The fix: Fill in all metadata fields before conversion—title, author, language, and publication date if available. In ebookconvert.pro, the Book Details step is where you enter this information. Don't leave fields blank, and avoid special characters in the author name (stick to standard letters and spaces).
6. Improper Handling of Footnotes and Endnotes
Word footnotes and endnotes sometimes convert to EPUB with broken links or missing content. The validator catches references that don't link to actual notes.
The fix: Before conversion, review all footnotes and endnotes in Word. Make sure they're inserted using Insert → Footnote/Endnote, not typed manually. If you have many notes, test a sample conversion first to ensure they render correctly in the EPUB preview.
7. Unsupported or Conflicting CSS Rules
You've applied complex formatting (unusual fonts, absolute positioning, or CSS that only works in web browsers). EPUB readers have limited CSS support, and validators reject rules that don't comply with EPUB 3 standards.
The fix: Stick to simple, semantic formatting: bold, italic, underline, and standard indentation. Avoid absolute positioning, complex tables, or custom fonts unless they're embedded properly. When in doubt, test your conversion in an EPUB preview tool before uploading to retailers.
How to Catch Validation Errors Before Publishing
The best time to fix validation errors is before you send your EPUB to retailers. Here's a practical workflow:
Step 1: Use a Free EPUB Validator
Before uploading anywhere, run your EPUB through a free validator:
- EPUB Check (by the IDPF): The industry standard. Upload your EPUB and get a detailed report of every validation error.
- W3C EPUB Validator: Another trusted tool that checks EPUB 3 compliance.
- Pagina EPUB-Checker: Detailed, user-friendly reports.
These tools give you a list of errors with line numbers and descriptions. Most errors fall into the categories above.
Step 2: Review the Error Report Systematically
Don't try to fix everything at once. Group errors by type:
- All metadata errors together
- All image/file reference errors together
- All link/navigation errors together
Fix one category, regenerate your EPUB, and validate again. This prevents you from chasing phantom errors caused by earlier mistakes.
Step 3: Go Back to Source (Your Word File)
Most validation errors originate in your Word manuscript. If your EPUB has broken links, check your Word headings. If images are missing, check your Word embedding. Fixing the source is faster than trying to patch the EPUB after conversion.
Tools like ebookconvert.pro include a free preflight step where you can review the detected chapter structure before conversion. This catches heading and structure issues before they become validation problems. Use it—it's one of the easiest ways to prevent errors downstream.
Why EPUB Validation Matters for Distribution
Retailers have different standards. Amazon KDP is relatively forgiving; Apple Books is strict. If your EPUB fails validation, you might:
- Face rejection from Apple Books or other premium retailers
- Experience formatting problems on certain reading devices (Kindle, Kobo, etc.)
- Have accessibility issues that block screen-reader users
- Waste time on support tickets instead of marketing your book
A valid EPUB is a distributable EPUB. Spending 30 minutes on validation before upload saves you hours of troubleshooting later.
Your EPUB Validation Checklist
Before you convert and publish:
- ☐ All chapter headings styled as Heading 1 (or consistent heading hierarchy)
- ☐ All images embedded (not linked) in JPG or PNG format
- ☐ No spaces or special characters in image file names
- ☐ Document language set correctly in Word
- ☐ Metadata complete (title, author, language)
- ☐ Footnotes and endnotes inserted via Word's built-in tools
- ☐ No custom fonts or unsupported CSS applied
- ☐ Internal links use Word's cross-reference feature
- ☐ EPUB validated with EPUB Check or W3C validator before uploading to retailers
The Bottom Line
EPUB validation errors are preventable. Most stem from inconsistent Word formatting, missing metadata, or embedded images. By cleaning up your manuscript before conversion and running a free validation check on your EPUB, you'll catch 95% of problems before they reach retailers.
The workflow is straightforward: prepare your Word file carefully, use a tool that gives you visibility into the conversion structure (like ebookconvert.pro's free preflight), generate your EPUB, validate it, fix any errors in the source, and regenerate. You'll ship a professional, distribution-ready file on the first try—no rejections, no re-uploads, no frustrated readers.
