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@roger64: That site is complete propaganda. Most of those percentages come from HTML renderer tests. Since all major readers (except possibly the one from Adobe) are based on WebKit engines, those percentages would be virtually identical if those same tests were carried out with the same HTML/CSS in an EPUB 2 wrapper. For instance, if you ran that silly test suite with the calibre ebook viewer you would get a pretty high percentage, despite the fact that calibre does not support epub 3, at all.
You dont seem to understand how ebook rendering works. An EPUB is just a zip file full of HTML/CSS. A ebook renderer basically unzips that zip file and feeds the HTML/CSS to a browser rendering engine, which then renders it into what you see on the screen. The contents of the opf are almost entirely meaningless to this process. No reader I know off deliberately disables features of its rendering engine based on the contents of the opf file. Therefore, any fancy epub 3 thing you like that works in your shiny epub 3 reader of choice, you can put into an epub 2 and it will still work with that reader.
The only thing in EPUB 3 that does not work in EPUB 2 is the new metadata. Of that, the only useful addition that I can see is series metadata. And I'm sorry, but implementing the entire epub 3 spec to get series metadata, which anyway nothing on the market currently supports and which already exists in epub 2 via a calibre extension, is just not worth it.
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