separator for adjacent footnote rendering

[glyph]

Right now, multiple footnote references rendered sequentially looks like this:

something with a footnote¹²³

Which would misleadingly lead the reader to believe that this is referencing footnote 123; a better rendering would be.

Because there is no CSS selector for "adjacent node with no non-whitespace text nodes between them" this is not really fixable with CSS, without creating stray commas or awkward extra spacing.

It would be better if it read like this:

something with a footnote^{1, 2, 3}

or at least put multiple a nodes inside the same sup node, so that CSS could rely on things like :last-child to determine where to put separators.