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