[Man Eating Duck]

Originally Posted by GraciousMe

Finally! I´ve found the code, here: http://www.w3.org/TR/html5/links.html#footnotes

[...]

Are my problems over now, or are they just beginning?

As others have said, those are HTML5-specific tags, which are generally not supported in epub 2. Although some HTML5 features will render fine in readers based on generic rendering engines like webkit, it likely won't work in the majority of dedicated readers. Sorry to disappoint you...

Originally Posted by GraciousMe

And this seems to be an even bigger question than footnotes: Should I stick to ePub2?

(The irony of calling me "Connoisseur"....)

You certainly should stick with epub2 if you want your epubs to be readable on a majority of readers. BTW, no one is a Connoisseur in these matters, we all have to cope with the mess that is the epub "standard" with various "embrace + extend" quirks, and a large amount of proprietary bugs, at least for a while. Not good.