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=Epub3 format=
https://www.epubzone.org/epub-3-overview/understanding-epub-3/
==Understanding EPUB 3==
This article is a high-level introduction to EPUB 3, the latest version of the EPUB format, a widely used and easily manipulable format for representing digital publications. It describes what EPUB is, why it’s needed, what kind of content EPUB is suited for and the many options for creating and distributing EPUB publications. If you’re an author, editor, or other publishing industry professional, and want to understand the rapidly emerging open standard for next-generation portable documents – without having to understand the details of what’s “under the covers” – you’re in the right place.
NOTE – Companion articles discuss means of making and distributing EPUB 3 publications – for the impatient who don’t care about the “what” and want to get straight on to the “how” you might want to start with one of these articles. A third companion article discusses the current adoption status of EPUB 3.
Even though EPUB publications are, like websites, amenable to hand-authoring and hand-tweaking (a core strength of the EPUB format), some people may appreciate an overview of EPUB from a functionality-oriented perspective (as one might cover PDF, JPEG, or ZIP functionality without delving into any of their internal file-format details). This article assumes that you have a very basic understanding of HTML and CSS, and that you know what JavaScript is.
TIP- If you prefer a more markup-centric introduction, the (free!) “What is EPUB 3?” from O’Reilly is highly recommended as it the overall O’Reilly “EPUB 3 Best Practices” book.
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==EPUB 3 In a Nutshell==
EPUB is a straightforward format. An EPUB file (extension: .epub) is simply a ZIP archive that contains what is in effect an entire website, including HTML files, all necessary images, CSS style sheets and other assets, plus additional structure and information (“metadata”) that makes the content able to be reliably consumed from beginning to end by any application or device that’s compatible with the EPUB specifications (“EPUB reading systems” or just “reading systems” for short).
EPUB 3 is the latest version of EPUB. EPUB 3 is based on the latest HTML5 standard, which means EPUB publications can now contain video, audio, and interactivity – just like websites in modern browsers.
An EPUB publication’s content is by default reflowable and most reading systems dynamically paginate that content. Adapting content display to the screens rather than forcing the reader to pan and zoom around pre-formatted content (and, more generally, making content accessible to different modes of consumption) is one of the key characteristics that distinguishes EPUB from PDF, a portable document format designed to represent print-replica content. But EPUB 3 can also, like PDF, represent fixed-layout, pre-paginated content. This can be useful for certain kinds of highly-designed content (illustrated children’s books, digital magazines, etc.) that are designed to be consumed only on a larger-screen device, such as a tablet.
NOTE – What makes EPUB different from a website in a ZIP file
To be more specific, and get only a bit more technical, an EPUB publication differs from an arbitrary website put into a ZIP archive in the following ways:
Manifest – an EPUB publication contains a data structure detailing all of its constituent resources – in effect, a formal and complete “packing list”
Defined reading order – an EPUB publication has a “spine” that determines a start-to-end reading order for its top-level content items (though nothing requires that the publication to be consumed in that order)
Standardized table of contents and related navigational structures, including the ability to define links into content fragments down to the sentence or word level
Metadata – publication and item level information about the publication and its components
XHTML – the HTML content in an EPUB publication must use the XML-based serialization of HTML5, which makes it directly manipulable by XML tools
Media Overlays – Well-defined publication-wide text+audio synchronization (an EPUB publication can be both an eBook and audio book)
Core Media Types (such as image and audio formats) and minimum CSS features are well-defined, along with a requirement for fallbacks when non-standard content types are used. This enables more reliable processing of EPUB publications, especially when distributed indirectly, than arbitrary websites which may use less standard content types without fallbacks and bleeding-edge CSS constructs that may not be universally supported.
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