Currently, we write out control chars to XML. That's invalid XML. Need to escape those. Can be in a feature tag, or name table, or CFF, etc. Test for example with Salsa-Regular.otf from googlefontdirectory.
It is possible for that font to have ttx create a line like:
which cannot be processed by other tools working on the XML data structure.
Please improve ttx so this will cannot happen. Either exit(1) or try to repair the data.
I'm not sure what we get with current code, specially it may be different between Python2 and Python3.
Reminder to self: see if we can define extra markup to escape invalid control chars in XML.
some relevant links: