What Is EPUB Validation and Why Should You Care?
If you've converted a Word manuscript to EPUB, you might think the job is done. But there's an invisible step that separates professional ebooks from ones that break on certain readers: EPUB validation.
EPUB validation is the process of checking your ebook file against the official EPUB 3 standard. Think of it like a spell-checker for code. It scans for structural errors, broken links, missing metadata, and formatting inconsistencies that can cause your book to display incorrectly—or fail to open at all—on Apple Books, Kindle readers, Kobo, or other platforms.
The problem? Most authors never validate their files. They upload to retailers and hope for the best. Then readers complain that chapters are out of order, images don't appear, or the book won't open on their device. By then, it's too late.
Common EPUB Validation Errors (and What They Mean)
Not all validation errors are created equal. Some are critical; others are warnings you can live with. Here are the ones you'll actually encounter:
Structural Errors
- Missing or malformed OPF file — The OPF (Open Packaging Format) is the spine of your EPUB. Without it, readers can't navigate your book.
- Missing or broken NCX file — This is the table of contents. If it's broken, readers can't jump to chapters.
- Unreferenced files — You've included images or CSS files that aren't linked in the content. They'll bloat your file size and serve no purpose.
Content Errors
- Broken internal links — Links to chapters, footnotes, or cross-references that point to sections that no longer exist.
- Missing alt text on images — Required for accessibility. Screen readers need descriptions of images.
- Invalid HTML markup — Unclosed tags, deprecated elements, or nesting errors that confuse readers.
Metadata Issues
- Missing title or author — Some retailers reject EPUBs without proper metadata.
- Invalid ISBN or date format — Metadata must follow specific formatting rules.
Font and Styling Problems
- Embedded fonts not declared — If you've embedded custom fonts, they must be listed in the manifest.
- CSS errors — Invalid properties or unsupported values that can break formatting.
The good news? Most of these errors are preventable or fixable if you catch them before publishing.
How to Validate Your EPUB File
The industry standard for EPUB validation is epubcheck, an open-source tool maintained by the EPUB community. If you're using ebookconvert.pro, you can upload your EPUB file directly to the validation tool, which runs epubcheck 5.1.0 and gives you a detailed report in seconds.
Here's how to validate your EPUB in three steps:
- Upload your EPUB file to a validation tool (or your conversion platform's built-in validator).
- Review the error report — It will list every issue, categorized by severity.
- Prioritize fixes — Fix critical errors first, then warnings, then informational notes.
When you validate early—right after conversion—you can catch issues while the original Word file is still fresh in your mind. This saves hours of debugging later.
How to Fix EPUB Validation Errors
Once you've identified errors, you have three options:
Option 1: Re-Convert Your Word File
If the errors stem from your original manuscript structure (missing chapter breaks, incorrect heading styles, orphaned images), the fastest fix is often to go back to Word, make corrections, and re-convert to EPUB.
Example: Your validation report says "Unreferenced image: cover.jpg." This usually means the image wasn't properly inserted into the manuscript, or it's floating outside the text flow. Fix it in Word, re-convert, and the error disappears.
Option 2: Manual Editing (Advanced)
EPUB files are actually just ZIP archives containing XML and HTML. If you're comfortable with code, you can unzip your EPUB, edit the files directly, and re-zip it. This works for:
- Adding missing alt text to images
- Fixing broken internal links
- Correcting metadata in the OPF file
- Removing unreferenced files
Tools like Sigil (free, open-source) make this easier by providing a graphical interface. But it requires patience and technical knowledge.
Option 3: Professional Fix Service
If you're not comfortable editing EPUB files yourself, or if the errors are complex, you can submit a fix request through your conversion platform. Many services, including ebookconvert.pro, offer an "Expert Human Fix" option where you describe the issue and a specialist corrects it for you. This is especially useful for structural problems that would take hours to debug on your own.
Best Practices to Avoid Validation Errors in the First Place
Prevention is always easier than cure. Follow these practices when preparing your Word manuscript:
- Use proper heading styles — H1 for chapter titles, H2 for sections. Don't just make text bold and larger.
- Insert images properly — Anchor them to text using "In line with text" or "With text wrapping." Avoid floating images.
- Use built-in styles for lists and quotes — Don't create custom formatting manually.
- Check for hidden formatting — Use Word's "Show/Hide" feature to reveal paragraph marks, tabs, and other invisible characters that can cause problems.
- Validate early — Convert a draft chapter first, validate it, and fix any systematic issues before converting the whole book.
- Include descriptive alt text for all images — Do this in Word before conversion, and it will carry over to your EPUB.
- Keep your metadata clean — Ensure your title, author, ISBN, and publication date are correct and properly formatted before conversion.
What Happens If You Ignore Validation Errors?
You might get away with it. Many readers won't notice minor issues. But here's what can happen:
- Retailers reject your submission — Apple Books and some other platforms automatically reject EPUBs with critical errors.
- Readers report bugs — Someone buys your book, it breaks on their device, and they leave a one-star review.
- Your book looks unprofessional — Even if it doesn't break, formatting issues make your book look rushed or poorly produced.
- Accessibility issues — Missing alt text and broken structure make your book unreadable for people using screen readers.
Validation takes 30 seconds. Fixing a bad reputation takes months.
The Validation Workflow: Start to Finish
Here's a practical checklist for validating and fixing your EPUB before publishing:
- Convert your Word manuscript to EPUB using your preferred tool.
- Run your EPUB through a validator (epubcheck-based).
- Review the error report and categorize by severity.
- For structural errors, go back to Word, fix the source, and re-convert.
- For content errors (broken links, missing alt text), decide whether to re-convert or manually edit.
- For metadata issues, update your manuscript metadata before re-converting.
- Re-validate after each fix until you have zero critical errors.
- Before publishing, do a final spot-check: open your EPUB in a few different readers (Apple Books, Kindle, Kobo simulator) and flip through it.
Final Thoughts
EPUB validation isn't glamorous, but it's the difference between a book that works and one that frustrates readers. The standard tool, epubcheck, has been the industry benchmark for over a decade. Whether you use a platform like ebookconvert.pro that includes built-in validation, or you validate manually, make it a non-negotiable part of your publishing workflow.
Your readers—and your reviews—will thank you.
