OPF-088 is reported as usage by default. Informative only and hidden by default: it appears when EPUBCheck runs with -u (--usage). It does not make the EPUB invalid.

EPUBCheck · Package document (OPF)

EPUBCheck OPF-088: unknown epub:type value

Updated: Nov 4, 2026

EPUBCheck OPF-088 is a usage message: “Unrecognized epub:type value "…".” An epub:type value is not in the EPUB structural semantics vocabulary, so reading apps will ignore it. It is a usage note (shown with -u) and does not make the EPUB invalid.

The official message

Unrecognized epub:type value "%1$s".

Default severity: Usage. Informative only and hidden by default: it appears when EPUBCheck runs with -u (--usage). It does not make the EPUB invalid.
EPUBCheck version: 5.4.0

%1$s, %2$s… are filled in with the file, value or element EPUBCheck found.

Typical causes

A misspelling such as "footnotes" on a single note.
A value invented for styling, such as epub:type="red-box".
A value from another vocabulary used without its prefix.

How to fix it

Read the value EPUBCheck quotes.
Use the correct value from the vocabulary (footnote, endnote, chapter, toc…) if one fits.
For styling, use class instead; for another vocabulary, give the value a declared prefix.

When it's not this

If the value exists but is deprecated, EPUBCheck reports OPF-086b.

Terms used here

Other OPF messages

OPF-089 The "alternate" link rel keyword cannot be paired with other keywords.
OPF-086 Property "…" is deprecated. Consider using … instead.
OPF-086b epub:type value "…" is deprecated. Consider using … instead.
OPF-086c the "…" reserved prefix is deprecated.
OPF-090 It is encouraged to use MIME media type "…" instead of "…".
OPF-085 "dc:identifier" value "…" is marked as a UUID, but is an invalid UUID.

Sources

EPUB 3.3, §C.2 The epub:type attribute · read Nov 4, 2026

Message text and severity from EPUBCheck 5.4.0 (W3C, BSD-3-Clause). Explanation by EpubCC.