# EPUBCheck HTM_061: … is not a valid custom data attribute…

Source: https://epublab.com/epubcheck/htm_061/ (updated 2026-11-03)

EPUBCheck HTM_061 is an error message: “"…" is not a valid custom data attribute (it must have at least one character after the hyphen, be XML-compatible, and not contain ASCII uppercase letters).” A data-* attribute name is not valid: it needs at least one character after the hyphen, no uppercase ASCII letters and XML-compatible characters.

- Official message (EPUBCheck 5.4.0): "%1$s" is not a valid custom data attribute (it must have at least one character after the hyphen, be XML-compatible, and not contain ASCII uppercase letters).
- Default severity: Error

## Typical causes

- Names such as data-, data-Page or data-x:y.

## How to fix it

- Rename the attribute in lowercase, for example data-page.

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