Maybe non-ASCII characters should be converted for text/html.
At least <FONT COLOR=...> should be possible inside of <nofill>...</nofill>. Presently all font changes in <nofill>...</nofill> are ignored, since <nofill>...</nofill> is converted to <PRE>...</PRE> and a </FONT> in <PRE>...</PRE> resets the font to a non-proprotional font.
A </color> in text/enriched should change the text color in text/html to the previous color rather than to the default color.
Have a look at some minor problems with text/enriched emails generated by exmh.
Got info that Eudora email clients now embed text/enriched text in <x-rich> and </x-rich>. This is not defined in RFC 1896.