Section 5.4, "Space Stuffing", I would add some words indicating that the
space stuffing is similar to dot-stuffing already present in MTAs, perhaps
a sentence like:
Space-stuffing is done by the sender and undone by the receiver, thus
causes no formatting changes to the message body.
It should also be mentioned what happens if a sender does space-stuffing
and the receiver doesn't know to undo space-stuffing - which would be
typical for all MUAs that exist today which don't understand
Format=Flowed. Such text might best belong in the Failure Modes section
(which might be renamed "Interoperability Issues" perhaps). I don't have
any text to recommend for this, though.
-Dan Wing