title_body: What can I do about Sigil breaking EPUB3 files
upvoted_answer: There is some hope on the horizon for the many users of EPUB3 files¹
  masayoshi takahashi has presented a relatively small pull request with changes for the Sigil source base that does prevent Sigil from breaking the OPF file when saving EPUB 3.
  
  Hopefully the next Sigil release will incorporate these. In the mean time you can of course compile your own Sigil to include these changes, try them out and report any issues if you encounter them.
  
  ¹ The style-sheet facilitates in EPUB 3 support the top-bottom written languages, most Japanese EPUB Ebooks are EPUB 3