An EPUB is a small website, zipped up with a spine.
Each chapter is a web page, the look comes from a stylesheet, and a manifest says what order to read them in. That's why EPUB text flows to any screen and why you can change the font. Open the folder of a short novel below.
<body> <h1>The Harbour</h1> <p>The ferry left at dawn. By the time the harbour lights went out behind them, …</p> <img src="images/harbour.jpg" alt="The harbour at dawn"/> </body>Each chapter is a standard web page. No page numbers or positions, which is why the text can reflow.
An EPUB is a ZIP, so on its own it starts like any other archive. The container rules add one trick: a file called mimetype must come first, stored uncompressed, with no extra field. Then the words “application/epub+zip” always sit at byte 38, and software can recognise the book without unzipping it. Break a rule and watch the signature move or vanish.
Every file must appear in the manifest, but only the spine sets what comes next when you turn the page. A spine entry marked linear="no" is skipped by page turns and reached only through a link, which is how endnotes and pop-up answers usually work. Reorder the spine and see what a reader does.
<spine> <itemref idref="cover"/> <itemref idref="toc"/> <itemref idref="c1"/> <itemref idref="c2"/> <itemref idref="c3"/> <itemref idref="notes" linear="no"/> </spine> <!-- manifest lists all 6 files, plus style.css and images; nav.xhtml carries properties="nav" -->
EPUB Accessibility 1.1 asks publishers to describe a book’s features in the package metadata, using schema.org terms. Bookshops and reading apps turn these into the accessibility summary you see before buying. Since the European Accessibility Act took effect in June 2025, most new ebooks sold in the EU need this information. Tick what the book offers.
<meta property="schema:accessMode">textual</meta> <meta property="schema:accessMode">visual</meta> <meta property="schema:accessModeSufficient">textual</meta> <meta property="schema:accessModeSufficient">textual,visual</meta> <meta property="schema:accessibilityFeature">alternativeText</meta> <meta property="schema:accessibilityFeature">tableOfContents</meta> <meta property="schema:accessibilityFeature">structuralNavigation</meta> <meta property="schema:accessibilityHazard">none</meta> <meta property="schema:accessibilitySummary"> All images are described. Headings follow the chapter structure. </meta>
An EPUB works when every reference inside it resolves. That means every link, image, stylesheet and font, plus the manifest and the navigation. This page parses each XHTML page and stylesheet with your browser’s XML parser and resolves every reference against the package. It checks fragment targets against the ids that really exist, compares each declared media type with the file’s first bytes, and works out which manifest properties each page needs. Then it writes a repaired copy and checks it again. Readers differ in how forgiving they are: a broken reference that one app ignores can stop another, or fail a shop’s upload check. Everything happens on your device.
Most readers open all three. The differences show up in accessibility, audio and page design.
Picking the right EPUB for a device means knowing which of three things you actually have. EPUB 2 carries its table of contents in an NCX file. EPUB 3 replaced that with an HTML navigation document and added support for audio, video and scripting. Fixed-layout EPUB 3 abandons reflow entirely and pins every element to a position, which is how comics, children's books and anything heavily illustrated are published.
The practical difference shows up as a missing contents list. An EPUB 3 file is not required to include the older NCX, and the page above reports when one is absent, but some older e-readers will show no table of contents at all without it. If a book opens on a newer device and looks unnavigable on an older one, that is usually the reason rather than a damaged file.
Fixed layout is the one to identify before converting. Because positions are pinned, the text cannot reflow to a smaller screen, so a fixed-layout book designed for a tablet is close to unreadable on a six-inch reader: you get the whole page shrunk rather than larger type. Converting it to a reflowable format means discarding the layout the publisher built, and converting it to PDF keeps the layout but keeps the screen-size problem with it.
The spine is worth a look for a different reason. It sets reading order, and entries can sit outside the linear flow, which is how a publisher marks material that should be reachable but not read in sequence, such as a copyright page or an appendix. A reader that honours that flag will skip those sections when you page forward; one that ignores it will drop you into them.
For device targeting, the rule of thumb is that reflowable EPUB is the safe default and anything else is a compromise. Send a reflowable EPUB 3 with an NCX included and almost every reader made in the last decade will open it with working navigation. Keep fixed layout for the books that genuinely need it, and read those on the screen size they were laid out for.
The EPUB a shop sells you and the EPUB this page can open are frequently different things. Stores wrap purchases in digital rights management, and a wrapped file is not readable by anything except the app licensed to open it. This converter detects those books and refuses them rather than producing a broken result, which is the honest outcome: the file is not damaged, it is locked, and no converter can lawfully unlock it.
Amazon's current format is the clearest case. KFX is proprietary and usually DRM-protected, so it cannot be read here at all. That is a deliberate boundary rather than a missing feature. If a book arrived from the Kindle store it is likely KFX or a protected AZW variant, and the route to reading it elsewhere runs through the retailer's own permissions, not through a file conversion.
Kobo adds a gentler wrinkle. Kobo devices read ordinary EPUB natively, so nothing needs converting for them, but the shop also sells a kepub variant. The difference is smaller than the separate extension suggests: kepub adds reading statistics and faster page turns and is otherwise the same book. If you have a choice, plain EPUB is the more portable file and loses nothing you would notice.
Books that are genuinely yours behave differently. An EPUB from a publisher's direct sale, a library of public-domain titles, or your own export has no wrapper, opens here, and converts cleanly. That is the file worth keeping as your master copy, because it will still open in a decade regardless of which retailer still exists, and every other format you might need can be generated from it.
The practical habit is to check what you have before planning around it. Open the book here and the package, spine and metadata are listed; a protected file says so immediately instead of failing halfway through a conversion. EPUB 3 itself is a W3C Recommendation and deliberately open, so where a book is unreadable the obstacle is almost always the shop's wrapper rather than the format.
Identifiers and rules from the W3C EPUB 3.3 Recommendation, which now defines the format, and the steps from Open eBook to today.
Each page explains what's kept and what changes.