Auto-Downgrade in Auto-Detect

Group CaseIDallHtmlallPlainPrefers=Unknown
in recipients 		ConvertibleCurrent Action Comment
allHtml (A-1) true false No Unknown Yes nsIMsgCompSendFormat::
HTML
(A-2) true false No Unknown No nsIMsgCompSendFormat::
HTML
allPlain (B-1) false true No Unknown Yes nsIMsgCompSendFormat::
PlainText		Auto-Downgrade
(B-2) false true No Unknown No nsIMsgCompSendFormat::
PlainText		Auto-Downgrade
Possible data loss for mail sender,
because Not-Convertible
(Note: Never for Text Domain recipent)
Mixed
(With Unknown)		(C-1) false false Unknown
is containd
in recipients		Yes nsIMsgCompSendFormat::
PlainText		Auto-Downgrade
Cause of mystery for user
If Unknown is interpreted as HTML,
changed to (A-1) or (D-1).
If Unknown is interpreted as Text,
changed to (B-1) or (D-1).
So, mistery disappers.
(C-2) false false Unknown
is containd
in recipients		No UseSendOptions
Mixed
(No Unknown) 		(D-1) false false No Unknown Yes nsIMsgCompSendFormat::
PlainText		Auto-Downgrade
Can produce same phenomenon as 'mystery for user'
but it's not mystery for user because reason of text/plain is understandable/acceptable for user
(Text Domain reipient is requested)
recipients=HTML+Text(no Unknown) is pretty rare in user's environment
(D-2) false false No Unknown No UseSendOptions
Note:
Convertible : Yes : aConvertible==Convertible::Plain, No : aConvertible==Convertible::No

What kind of simple HTML can produce Not-Convertible?

Any HTML falls into a column of following Table.
If Not-Convertible is returned to "tag which has style attribute",
probability of that 'mystery for user' occurs is drastically changed to low in 'CSS Style is used' cases of Simple HTML cases.

Repeated "severe/critical/massive format/data loss" seems "CSS data is lost in Type-1-2 case of the Table".

HTML type used HTML tag No CSS CSS Style is used
Simple HTML-A
(Type-1)		Body Text, <P>, <PRE> only Typ-1-1
O		Typ-1-2
O
Simple HTML-B
(Type-2)		Type-1 + <TT>, <A> with link text==link urlTyp-2-1
O		Typ-2-2
O
Complex HTML
(Type-3) 		Tag for format is used Typ-3-1
X		Typ-3-2
X

Consideration

  1. Defiition of "Text Domain" looks changed by Prefers=Text.
    Originally, Text Domain==unable to show HTML, or rejects HTML.
    However, for some users, Prefers==Text is "Recipiet prefers Text although mailer/device can show HTML.
    and, for some users, Prefers=Text and Text Domain is absolutely different entity.