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EPUBCheck · Resources and references

EPUBCheck RSC-032: foreign resource, no fallback

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EPUBCheck RSC-032 is an error message: “Fallback must be provided for foreign resources, but found none for resource "…" of type "…".” A resource in a format reading apps are not required to support (a foreign resource) is used without a fallback in a core format. Without a fallback, apps that cannot display it show nothing.

The official message

Fallback must be provided for foreign resources, but found none for resource "%1$s" of type "%2$s".

Default severity
Error. The EPUB breaks a rule of the specification and is invalid. Stores and Send to Kindle may reject it, and EPUBCheck exits with status 1.
EPUBCheck version
5.4.0

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

Typical causes

  • Images in TIFF, BMP or another non-core format referenced directly from the content.
  • Embedded objects or media in formats outside the core media types list.

How to fix it

  1. Convert the resource to a core media type (for images: JPEG, PNG, GIF, WebP or SVG).
  2. Or provide a fallback: a manifest fallback attribute, or a picture or object element with an alternative.
  3. Update the manifest media type and validate again.

When it's not this

An img pointing at a non-core image format can also be reported as MED-003.

Terms used here

Other RSC messages

  • RSC-031 Remote resource references should use HTTPS, but found "…".
  • RSC-033 Relative URL strings must not have a query component, but found one in "…".
  • RSC-030 File URLs are not allowed in EPUB, but found "…".
  • RSC-034 Scripts must have a JavaScript media type.
  • RSC-029 Data URL is not allowed in this context.
  • RSC-035 "…" is a legacy media type for JavaScript. Consider using 'text/javascript' instead.

Sources

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

FAQ

What does EPUBCheck RSC-032 mean?

EPUBCheck RSC-032 is an error message: “Fallback must be provided for foreign resources, but found none for resource "…" of type "…".” A resource in a format reading apps are not required to support (a foreign resource) is used without a fallback in a core format. Without a fallback, apps that cannot display it show nothing.

Is RSC-032 an error?

RSC-032 is reported as error by default. The EPUB breaks a rule of the specification and is invalid. Stores and Send to Kindle may reject it, and EPUBCheck exits with status 1.