# EPUBCheck OBS-001: Usage of … is outdated

Source: https://epublab.com/epubcheck/obs-001/ (updated 2026-11-03)

EPUBCheck OBS-001 is a usage message: “Usage of … is outdated.” The EPUB uses a feature that still works but is outdated, such as an NCX, font obfuscation, a prefixed CSS property or a deprecated element; the message names it.

- Official message (EPUBCheck 5.4.0): Usage of %1$s is outdated.
- Default severity: Usage

## Typical causes

- Books made for EPUB 2 or older reading systems, or CSS with vendor prefixes such as -webkit-.

## How to fix it

- Nothing breaks today. When you next update the book, replace the feature with its current equivalent (for example, unprefixed CSS properties).

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