In order to check that your EPUB is complete and ready for submission to retailers we can pass any EPUB created using CircularFLO to the EPUB validation tool Pagina EPUB-Checker. If present on your Mac and the option to in the Preferences Panel (Coming Soon!) then CircularFLO will use this tool to validate your EPUBs as they are created..
This validation tool uses Java which is no longer bundled with macOS and so to use this tool we require a one time installation of Java. This is freely available from here and this is the same page that Apple also direct users to in order to get Java installed.