Do you really need font support for that, though? I've found editor support for it much cleaner -- it knows when it's best to expand then and when they should be ligatured.
Yes! Someone has to make the glyphs and someone has to write the rules that say "unconditionally replace this sequence of glyphs with this glyph" or "in this context, replace this glyph with that one" and so on. The only thing the editor has to do is decide to use them.
They're usually grouped into features: standard ligatures (liga), discretionary ligatures (dlig), historical ligatures (hlig), contextual ligatures (clig), and so on. OpenType fonts can also provide stylistic sets, which might swap out some glyphs with alternatives.
My understanding is that both font and editor must support it; the editor must know when/how to render the ligatures, but the font must define what the glyphs look like, or else they won’t look any different that the regular sequence of characters.
I’m far from an expert, though; I’ve just gotten so used to having ligatures over the past few years that I now find it uncomfortable not to have them.
