Auto-Downgrade in Auto-Detect

[1] Mystery for user when Auto-Detect is used in HTML mail composition

Following phenomenon can occur in Thunderbird.

When Delivery Format=Auto-Detect is used(or forced) when mail is composed in HTML mode,
- If HTML is simple HTML(tag for formatting is not used),
  and if Thunderbird considers that the simple HTML is Convertible(to Text),
- and If recipients type is (!allHtml&&!allPlain),
  (usually, recipient is Prefers=Unknown recipent case)
the simple HTML mail is silently sent as text/plain by Thunderbird with UNKNOWN reason for user.

[2] Auto-Downgrade in Auto-Detect

Group	CaseID	allHtml	allPlain	Prefers=Unknown in recipients	Convertible	Current Action	Comment
allHtml	(A-1)	true	false	No Unknown	Yes	nsIMsgCompSendFormat:: HTML
allHtml	(A-2)	true	false	No Unknown	No	nsIMsgCompSendFormat:: HTML
allPlain	(B-1)	false	true	No Unknown	Yes	nsIMsgCompSendFormat:: PlainText	Auto-Downgrade
allPlain	(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.
Mixed (With Unknown)	(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
Mixed (No Unknown)	(D-2)	false	false	No Unknown	No	UseSendOptions
Note: Convertible : Yes : aConvertible==Convertible::Plain, No : aConvertible==Convertible::No

[3] What kind of simple HTML can produce 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".

What kind of simple HTML can produce Convertible=Yes?
HTML type	used HTML tag	No CSS	CSS Style is used
Simple HTML-A (Type-1)	Body Text, <P>, <PRE> only	Type-1-1 O	Type-1-2 O
Simple HTML-B (Type-2)	Type-1 + <TT>, <A> with link text==link url	Type-2-1 O	Type-2-2 O
Complex HTML (Type-3)	Tag for format is used	Type-3-1 X	Type-3-2 X
Minimum "Simple HTML" example: <html><body bgcolor="#FFFFFF" text="#000000"> Type text here </body></html> Minimum "Complex HTML" example: <html><body bgcolor="#FEFEFE" text="#000000"> Type text here </body></html>

[4] Consideration

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.