Maybe we should do this across the board. We also recommend file extensions for the package document, svg, pls lexicons and smil files.

Maybe we should do this across the board. We also recommend file extensions for the package document, svg, pls lexicons and smil files.

Yes!

Ya, I wanted to remove these while retooling the conformance sections, but didn't want to get bogged down in a conformance change. I'll open a PR.

Proposed Solution

Remove the recommended file extensions for Package Documents, XHTML Content Documents, PLS Lexicons and Media Overlay Documents.

Add a note in the package document definition section to refer to the media type registration for more info about file properties.

Did some testing. Made the "Ivan nightmare" file where an XHTML content document used the file extension .svg. The resulting EPUB gives a warning in EPUBCheck, of course, but works in:

1. Apple Books
2. Calibre
3. Adobe Digital Editions 3.0
4. Thorium

Interestingly, when I tried to convert the EPUB to a non-EPUB format, the conversion tool complained.

Epubcheck doesn't inspect the content to verify the media type, so it's the easiest to fool. Faking media types was a periodic suggestion in the idpf forums for avoiding fallback requirements.