The footnote is the conscience of the page. It is where the writer admits, in a small voice, that someone else got there first; or that the claim is contested; or that the citation is partial; or that the reader, if she wishes, may verify. Pages without that small voice are pages without conscience.
We will use four kinds of note: the citation, which gives the source1; the qualification, which limits the claim2; the aside, which honours an objection3; and the cross-reference, which points to a sibling argument elsewhere on this page4. Each is set in Source Serif 4 at a smaller size, and ruled off by a thin line above.
It is no defense of one's claim to surround it with a great wall of citations. The wall must be load-bearing. A footnote that does not change the reader's confidence in the claim is a footnote that should not be there. Decoration in the apparatus is the surest sign of decoration in the argument.
We have, accordingly, instructed our contributors to footnote sparingly and well. The page is better for it. The reader is better for it. The argument, when it survives, is better for it.