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Glossary

What is EPUB core media types?

Updated:

Core media types are the file formats every EPUB 3 reading system is expected to support, so they need no fallback: GIF, JPEG, PNG, SVG and WebP images, MP3, AAC and Opus audio, CSS, TrueType, OpenType, WOFF and WOFF2 fonts, and XHTML. EPUB 3.3 lists them in a table.

Why it matters when you convert a PDF to EPUB

Images extracted from a PDF can be in formats that are not core media types, such as JPEG 2000 or TIFF-like encodings. A converter has to re-encode them into a core type, usually JPEG or PNG, or the EPUB needs a fallback for each one and some readers will show nothing.

A file in a format outside the list is a foreign resource, and EPUBCheck reports missing fallbacks for it (RSC-032) or an image element that points to it (MED-003).

How EpubCC handles it: every image the engine keeps is written as JPEG or PNG, so the EPUB contains only core media types and never needs a fallback chain.

Related terms

EPUBCheck messages about this

  • MED-003 Picture "img" elements must reference core media type resources, but found resource "…" of type "…".
  • RSC-032 Fallback must be provided for foreign resources, but found none for resource "…" of type "…".

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FAQ

What is EPUB core media types?

Core media types are the file formats every EPUB 3 reading system is expected to support, so they need no fallback: GIF, JPEG, PNG, SVG and WebP images, MP3, AAC and Opus audio, CSS, TrueType, OpenType, WOFF and WOFF2 fonts, and XHTML. EPUB 3.3 lists them in a table.

Is WebP allowed in EPUB?

Yes. EPUB 3.3 lists image/webp as a core media type, but older reading systems that predate it may not display it, so JPEG and PNG remain the safest choices.