Stef -- you are absolutely right that the prose I last posted was
insufficiently user-focused. How about replacing the last paragraph as
follows:
Whether or not text is justified by default (that is, whether the
default environment is flushleft, flushright, or flushboth) is
unspecified, and depends on the preferences of the user, the
capabilities of the local software and hardware, the nature of the
character set in use, and the tastes of the implementors. On systems
where justification is considered undesirable, the flushboth environment
may be identical to the default environment. Note that justification
should never be performed inside of center, flushleft, flushright, or
nofill environments. Note also that for some non-ASCII character sets,
full justification may be fundamentally inappropriate.