EpubCCWhat is an EPUB reading system?
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An EPUB reading system is the software that opens an EPUB and presents it to a reader: an e-reader's built-in app, Apple Books, a Kobo or a phone app. EPUB 3.3 defines it as a system that processes EPUB publications for presentation to a user.
Why it matters when you convert a PDF to EPUB
A converted PDF is only as good as it looks in the reading systems people really use, and they differ: some show footnotes as pop-ups, some ignore landmarks, some apply their own margins and fonts over the book's CSS. Writing plain, valid EPUB 3 with few styling assumptions is the way to look reasonable in all of them instead of perfect in one.
The rules reading systems follow are in a separate W3C Recommendation, EPUB Reading Systems 3.3, which says how they must process the container, the package document, content documents and fixed layouts.
How EpubCC handles it: the engine writes the same simple structure for every book (sections, headings, paragraphs, footnotes as asides, a short stylesheet) so that the result does not depend on one app's extensions. The preview on the website shows a chapter in a browser frame, which is close to, but not the same as, your own reader.
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