Hans, thanks for the detailed answer! My understanding is slowly advancing :D
in principle we could do without, but this helps predefining a couple of things that otherwise would slow down each font switch
This confirms my (vague) assumption that deep in the internals it helps structure things.
it also relates to bodyfont environments where there are relationships between sizes
... thereby creating the a "font enviroment"
in practice, if you choose say 13pt it will work out ok as things are checked and defined on the fly; but there might be a slight performance hit
This goes towards my fear: I prefer to state things explicitly. "Automagic" that happens in the background covering up for my sloppyness is great, but... creates confusion when things trip up. Internals might change. Or I might have been building on side effects for all I know. For this reason examples with wordy declarations are welcome... Working with hard-blocking Modula compilers was a good teacher, but that's long ago.
you probably want features enables, so "*default" \definefont [WonderFont] [BoldItalic*default sa 4.8]
Hm, I'll have to read up on that one. Thanks ! Martin