OPF-037 is reported as warning by default. The EPUB is still valid, but something is likely to cause problems in some reading apps. EPUBCheck exits with 0 unless you pass --failonwarnings.

EPUBCheck · Package document (OPF)

EPUBCheck OPF-037: deprecated media type

Updated: Nov 4, 2026

EPUBCheck OPF-037 is a warning message: “Found deprecated media-type "…".” An EPUB 2 manifest uses a deprecated media type, which EPUBCheck names. In EPUB 2 these are the old OEB types such as text/x-oeb1-document, kept only for compatibility with very old publications.

The official message

Found deprecated media-type "%1$s".

Default severity: Warning. The EPUB is still valid, but something is likely to cause problems in some reading apps. EPUBCheck exits with 0 unless you pass --failonwarnings.
EPUBCheck version: 5.4.0

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

Typical causes

A package document carried over from an OEBPS 1.x publication.
A tool that still writes the old OEB media types.
Manifests copied from very old books.

How to fix it

Find the manifest item EPUBCheck points to.
Change text/x-oeb1-document to application/xhtml+xml, and text/x-oeb1-css to text/css.
Make sure the files really are XHTML and CSS; old OEB documents may need cleaning before they validate.

When it's not this

If the deprecated type is text/html, EPUBCheck reports OPF-035 instead.

Terms used here

Other OPF messages

OPF-036 Video type "…" might not be supported by reading systems
OPF-035 Media type "text/html" is not appropriate for XHTML/OPS.
OPF-034 The spine contains multiple references to the manifest item with id "…".
OPF-040 Fallback item with id "…" could not be found.
OPF-033 The spine contains no linear resources.
OPF-041 Fallback-style item with id "…" could not be found.

Sources

EPUBCheck 5.4.0 message texts (MessageBundle.properties, w3c/epubcheck on GitHub) · read Nov 4, 2026

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