This Twitter thread
from earlier in the week (about the Component class in GDEF, and whether it really is used) started me thinking — there are a number of elements in the OpenType specification that are pretty widely recognized as being "not practically implementable" for one reason or another, such as lack of detail or, perhaps, because they were a little aspirational
when they were first added, and things went a different direction....
For example, the JSTF table or the `rand` feature. But they tend to emerge in discussions rather arbitrarily & unpredictably when someone bumps into them.
So I wonder: is there a somewhat-current list somewhere of those bits & gaps? The things that are listed in the spec, but cannot actually be supported IRL? Other than digging through HarfBuzz source code or font-editor documentation, that is.
Or, if there's not such a list, then alternatively please feel free to chime in with replies and perhaps have a casual one emerge below....
(I do realize that there's a lot of personal interpretation in calling something "impractical" or "not implementable"; I'm totally fine with those caveats being assumed and nobody will take such a comment as being official or definitive. Unless you want to make a case for yours.)