title_body: ePub converter that saves in HTML5? Does anyone have an ePub converter that actually saves in HTML5? 
  I have tried Kotobee Author and Calibre but they save in HTML 4/XHTML. An update of the Mozilla Composer/SeaMonkey IDE (Internet Developer Environment) came out in Feb. 2018 but that also saves in HTML 4. 
  A lot of IDEs and ePub apps claim to support HTML5 but save to HTML 4.
  Any suggestions?
  I don't want to manually migrate my code to HTML5 and then have the ePub converter wrap it as HTML 4. Or am I making too big a deal of this?
  Thanks, in advance.
upvoted_answer: Sigil supports both epub2 and epub3 books and one of the Sigil developers has published a plugin that'll convert epub2 books to epub3 books.