Simon Pepping wrote:
Aren't ligatures a feature of the font,
Yes and no. While the font may provide a glyph, it is the responsibility of the content rendering code to decide whether a ligature should be used. Deciding whether a ligature is applicable is not necessarily trivial, for example there is usually a ligature for the "ffi" character triple (especially for slanted fonts), but the word "auffinden" uses only the "fi" ligature because "auf-" is a prefix for the verb "finden". The oe ligature in swedish (or danish) has even weirder rules. The renderer may try to fake a ligature if the font doesn't have a glyph, or it may just ignore the issue.
NFC is the standard for the web. Does that carry any weight?
Given that this choice is buried deep in the FOP engine, far away from any interface intended for average users, I'd say we can choose whatever we think suites best. It's only a concern for hyphenation pattern generation and the FOP dev team. J.Pietschmann