OpenType mark & mkmk features: building, testing, and inspecting
It can be tricky to get 'mark' and 'mkmk' features working in a font, but they are useful for allowing users to typeset characters that you didn’t think of when making pre-composed diacritics. It can also be kind of tough to know how to test these features once they are added, and how to inspect what feature code is actually in a binary font file. Here’s a quick, casual run through of how to do each of these things.