# EPUBCheck HTM_054: reserved custom namespace, how to fix it

Source: https://epubcc.com/epubcheck/htm_054/ (updated 2026-11-04)

EPUBCheck HTM_054 is an error message: “Custom attribute namespace ("…") must not include the string "…" in its domain.” A custom attribute uses a namespace whose domain contains w3.org or idpf.org. EPUB allows prefixed custom attributes only in namespaces outside those two domains, which are reserved for the standards themselves.

- Official message (EPUBCheck 5.4.0): Custom attribute namespace ("%1$s") must not include the string "%2$s" in its domain.
- Default severity: Error

## Typical causes

- An attribute in a made-up namespace under w3.org or idpf.org, often a typo for a real namespace.
- A real EPUB or HTML attribute written with the wrong namespace URI, so it is treated as a custom one.

## 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. If the attribute is a standard one (such as epub:type), declare its correct namespace, for example xmlns:epub="http://www.idpf.org/2007/ops".
3. If it is your own or a vendor's attribute, move it to a namespace URI on a domain you control, or remove it.
4. Rebuild the EPUB and run EPUBCheck again to confirm the message is gone.

## When it's not this

If the problem is a data-* attribute name, that is HTM_061.

## Sources

- EPUB 3.3, §6.1.3.5 Custom attributes: https://www.w3.org/TR/epub-33/#sec-xhtml-custom-attributes (read 2026-11-04)

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