Quote:
Originally Posted by kovidgoyal
The only thing in EPUB 3 that does not work in EPUB 2 is the new metadata.
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Actually, if you have HTML in your EPUB that is 100% compliant with the appropriate standard (EPUB2 or EPUB3), then EPUB2 loses support for CSS 3, and at least some HTML.
But, most EPUB2 readers use an HTML/CSS renderer that can handle the features that are technically only EPUB3-compliant, so for real-world reading, there isn't really much difference. You can convert an EPUB3 to EPUB2 and have it render the same on most readers. But, you can't have an EPUB2 as a source that has those features. So, EPUB3 in ebooks is a good thing, but having Calibre support it other than to be able to convert to EPUB2 isn't worth the programming time.