For Web formats (HTML, etc.), the answer is "no".
The obvious follow-up to the list: It'd be interesting to know where the
answer is "yes".
People will occasionally mention ISO/IEC 2022, which can be thought of
as a meta-encoding or encoding template or encoding constructor, but in
the normal case a sensible position is that a document making use of
multiple encodings is no longer plaintext ("single text document" in
this thread's subject line). And – yes – "plaintext" is a fuzzy notion
around the edges, as others have successfully argued in the past.
Stephan
