[cedhax]

If an EPUB reader app solely relies on NCX, then it does not have basic support of EPUB 3. NAV document is considered the basic stuff of EPUB 3. Note that I am not talking about multimedia support, MathML, vertical writing modes, right-to-left page prrogression, etc., which you may consider as "advanced" features. But NAV for Table of Contents also "advanced" (and thus not basic)? Then what else can be considered "basic support"?

Opening an EPUB book with both EPUB2 and EPUB3 features but is designed to also work with EPUB2 readers for backward compatibility makes commercial sense, but this does not automatically qualify the EPUB2 readers as having EPUB3 basic support. You see, such a qualification process would render the term "EPUB3 basic support" pretty much meaningless--all you get is a hype.

I don't know which readers you consider as "mainstream". But I definitely consider iBooks as "mainstream". It does take full advantage of EPUB 3 nav. On this feature alone, I do consider iBooks has basic support of EPUB3. In fact, it has a lot more than basic support.