[skreutzer]

Yes, that's right, I'm affected by it because footnotes is something I'm using quite frequently. In theory, maybe one can place backlinks in the footnote text as fallback [1], but I haven't tried this yet. As of EPUB3 structure, I think there are only minor advantages and minor disadvantages compared to EPUB2, so I at least attempt to provide EPUB3 too as some kind of encouragement for software and device producers to gradually switch to EPUB3. Especially (optional) JavaScript support may open entire new worlds of what can be done with electronic offline texts (think of freely licensed libraries that could be incorporated, which work on semantic markup within the text).

[1] However, such kind of upward compatibility may always be an issue, an EPUB2 reading system therefore could just refuse to accept EPUB3 input, or ignore EPUB3 specific features, or attempt an best-effort approach, but of course there's no safe way to predict the future in the implementation, so only a firmware update might do.