# EPUBCheck OBS-001: outdated feature in use, how to fix it

Source: https://epubcc.com/epubcheck/obs-001/ (updated 2026-11-04)

EPUBCheck OBS-001 is a usage message: “Usage of … is outdated.” The EPUB uses a feature that still works but is outdated. EPUBCheck names it: an NCX, font obfuscation, a manifest content fallback, a deprecated element or property, or a prefixed CSS property or value. It is a usage note.

- Official message (EPUBCheck 5.4.0): Usage of %1$s is outdated.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 that keep an NCX or a guide.
- CSS with -epub- or vendor prefixes such as -webkit-, copied from older templates.

## How to fix it

1. Nothing breaks today, and you can leave it if older reading apps still matter to you.
2. When you next update the book, replace the feature with its current equivalent, for example unprefixed CSS properties such as writing-mode instead of -epub-writing-mode.
3. Keep an NCX only if you need EPUB 2 reading systems; EPUB 3 apps use the nav document.

## When it's not this

If the element is discouraged rather than outdated (base, rp, embed), the note is HTM_055.

## Sources

- EPUB 3.3, §A.2 Deprecated features: https://www.w3.org/TR/epub-33/#deprecated (read 2026-11-04)
- EPUB 3.3, §6.3.1.3 Prefixed properties: https://www.w3.org/TR/epub-33/#sec-css-prefixed (read 2026-11-04)

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