In an ebook I bought, if you click on the number of a footnote the text of the note appears as a popup, and you may choose to close the popup or go to the chapter with footnotes. I noticed this behavior both on Moon+Reader and in Calibre reader, so I am confident it depends from the raw text of the epub.
However, I looked at the source code and I did not find anything hinting at this. The source for a footnote is
<span class="apice"><a href="../Text/base7.html#fnref15" id="fn15">[15]</a></span>
The text for the note starts with
<p class="Note"><span class="apice-note"><a href="../Text/base4.html#fn15"
id="fnref15">[15]</a></span> blah blah ... </p>
and the relevant CSS is
p.Note {
font-size : 0.75em;
line-height : 1.2;
text-align : left;
color : #000000;
text-indent : 0px;
margin : 0px 0px 7px 0px;
}
span.apice {
font-weight : normal;
font-style : normal;
font-size : 0.70em;
vertical-align : super;
color : #000000;
}
span.apice-note {
font-weight : normal;
font-style : normal;
font-size : 0.80em;
vertical-align : super;
color : #000000;
}
Where is the trick?
Addendum: It seems indeed that I casually picked two of the very few reader applications which support footnotes - see this announcement on Facebook for Moon+ Reader and this thread for Calibre.