Quote:
Originally Posted by DoctorOhh
I may be wrong, but I don't believe that just because something is "not accepted by EpubChecker" means it is not epub3 compliant.
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True.
But I believe calibre's editor only "understands" ePub 2 when it comes to automatically updating/altering things in the opf file (metadata/file names/folder structure). So allowing calibre to automatically rebuild the opf to reflect those automatic changes will probably result in the opf being parsed/rebuilt as ePub2 (at least the package version declaration; which could make some meta entries invalid).
If the OP wants to use calibre's editor to update the metadata of a valid ePub3, they'd have to turn off the automatic metadata updating (like Kovid mentioned), and change the metadata manually in the OPF file. With "automatic metadata updating" turned off, manual changes to the markup and opf/ncx files should not result in the opf file being rebuilt.
I've manually edited valid ePub3s with calibre's editor and had them remain valid after saving. So it can be done. You just have to be careful and not let the editor do anything FOR you (except maintain the container, of course
).