In my experience, if you remove ligatures that exist in a 'calt' table and do not remove the corresponding 'calt' table entry, most font renderers will mindlessly display a rectangular box or a space where the character should be (i.e. "b." on Jellby's list). YMMV.
In other words, if you are subsetting a font, you
must do one of the following things:
- Read in the 'calt' table and painstakingly go through all the glyphs to see if any character patterns match the pattern on the left (comparison) side, and if so, add the glyphs on the right (output) side to the list of glyphs to keep,
- Read in the 'calt' table and keep any glyph that appears on the right (output) side no matter what, or
- Strip out any 'calt' table entries where the glyph on the right (output) side is being stripped out, which in practice probably means stripping out the 'calt' table entirely.
#1 is most correct. #2 is also correct but results in slightly larger files. #3 is kind of lame, because it will probably strip out all contextual alternates, but at least it won't result in missing letters in your text.