Comment 6 for bug 603309

The following also applies to Bug 793074:

Yes, it's physically possible to kern every character against every
other character - the problem is that OpenType has a limit on the
maximum number of traditional kern pairs (in the "kern" table), so as a
character set gets larger you have to become increasingly selective
about which pairs you include, and which you ignore.

The solution is, of course, class-based kerning using GPOS, which allows
perfect kerning of everything against everything else, with minimal file
overhead, but that requires the text layout library to "do the right
thing" with the OpenType features.

Kerning is glyph-against-glyph, not character-against-character, so
alternate characters, substitutions etc. can still be handled properly.

Dave