By far the largest[1] user of footnotes (both for citations, and supplemental information or commentary, online, is Wikipedia. You might like to look at how footnotes are done there[2], using popups and bi-directional links. Examples are available[3][4].

[1] {{citation needed}}

[2] There is more than one method in use

[3] https://en.wikipedia.org/wiki/William_Henry_Jackson_(priest)

[4] https://en.wikipedia.org/wiki/William_Booth_(forger)

[5] Beware orphan footnotes