# EPUBCheck HTM-010: unusual epub namespace, how to fix it

Source: https://epubcc.com/epubcheck/htm-010/ (updated 2026-11-04)

EPUBCheck HTM-010 is a usage message: “Namespace "…" is unusual for the prefix "epub".” The epub prefix is bound to an unusual namespace, so epub:type and other epub attributes may be ignored. Reading apps only recognise these attributes in the namespace http://www.idpf.org/2007/ops; with any other URI they are just unknown attributes. EPUBCheck reports it as a usage note and shows the namespace found.

- Official message (EPUBCheck 5.4.0): Namespace "%1$s" is unusual for the prefix "epub".
- Default severity: Usage

## Typical causes

- A typo in the namespace declaration, such as a missing slash or an https URL.
- A namespace copied from another vocabulary or an older draft.

## How to fix it

1. Open the file EPUBCheck names (the message gives the path, line and column) in a text or EPUB editor such as Sigil, or unzip the EPUB and open it there.
2. Declare the prefix on the html element exactly as xmlns:epub="http://www.idpf.org/2007/ops".
3. Keep the same declaration in every chapter so footnotes and other epub:type semantics work everywhere.
4. Rebuild the EPUB and run EPUBCheck again to confirm the message is gone.

## When it's not this

If a namespace is declared and simply never used, that is HTM-044, not HTM-010.

## Sources

- EPUB 3.3, §6.1.3.1 Structural semantics: https://www.w3.org/TR/epub-33/#sec-xhtml-structural-semantics (read 2026-11-04)

Message text and severity: EPUBCheck (W3C, BSD-3-Clause). Explanation: EpubCC.