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What are the differences between EPUB2 and EPUB3? Which is most popular?

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According to this page, EPUB3 adds, changes or improves:

  • Added HTML5 support
  • SVG documents can now appear in the spine in EPUB 3
  • Support for MathML
  • Semantic Inflection
  • Content switching has been simplified by having its processing model defined so that it does not require document preprocessing
  • Navigation
  • Linking
  • Scripting
  • Triggers
  • Bindings
  • Added modules from CSS3
  • EPUB 3 requires Reading Systems to support the OpenType and WOFF font formats for embedded fonts in conjunction with the CSS @font-face rules.
  • Audio and video
  • Media overlays
  • Publication Metadata and Identity
  • Resource Metadata
  • Text-to-speech
  • Manifest Fallbacks
  • Remote Resources
  • Whitespace in MIMETYPE file
  • Disallowed characters in OCF list has been extended

Things that have been removed:

  • DTBook
  • Out-of-Line XML Islands
  • Tours
  • Filesystem Container
  • Guide
  • NCX
  • 2.0.1 meta element
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From a production standpoint, there aren't really that many differences at a bare-bones level--there's the switch from toc.ncx to toc.xhtml to provide navigational information, a few new metadata elements required in the content.opf file (like dc:terms modified, which indicates when the file was last modified), and a new header to use. While you can leave out the old toc.ncx and still get a valid epub 3.0 file, for backwards compatability it's best practice to include both toc files--epub 3 readers will ignore the old one, and epub 2 readers will ignore the new one.

As far as bells and whistles go, there are some neat new things: HTML 5 support, including things like <canvas> and Javascript support, <video>, and <audio>. Fixed Layout is a pretty big deal, of course. MathML is great if you're doing anything that involves equations, and you can use epub:type to include semantic information that reading systems can do some neat things with (like the popup footnotes in iBooks).

Popularity of the formats is a bit of a non-issue; epub 3.0 files are viewable on every epub 2.0.1 reading system and device that I've tried them on, and I've been making epub 3.0 files for over two years at this point. The new additions that epub 3.0 bring don't work on the older devices, of course, but the books themselves don't break and are perfectly readable, even if the videos and javascript don't show up. Older epub files are, as you would expect, readable on the new reading systems out there. There's no reason not to produce epub 3.0 files--doing so will only add functionality to your books as more reading systems catch up to epub 3.

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