Re: [whatwg/encoding] Concatenating two ISO-2022-JP outputs from a conforming encoder doesn't result in conforming input (#115)

I am only one of the Thunderbird users in Japan. Please allow me to comment here.

In Japan, many users still send e-mail with ISO-2022-JP encoding.
Many Japanese Windows users usually use Shift_JIS instead of Unicode when dealing with Japanese texts. UTF-8 or UTF-16 is used only when it is necessary to handle characters not included in Shift_JIS.
However, sending 8-bit Shift_JIS text directly via e-mail caused problems.
For that reason, we selected ISO-2022-JP encoding that can express characters with 7 bits when sending Japanese e-mails.
This old method is still in use today.

Recently Thunderbird has been automatically updated to Version 60, however, cases where U + FFFD is inserted in the subject display of some e-mails later have been reported since then.
For example, it was [posted to MozillaZine.jp](https://forums.mozillazine.jp/viewtopic.php?f=3&t=17271), and [a bug report was posted to bugzilla.mozilla.org](https://bugzilla.mozilla.org/show_bug.cgi?id=1506049).
However, this bug report became RESOLVED because Firefox(and Thunderbird) conforms to the spec. That is why I came here.

Although there is no concrete report, I think that the e-mail software that performs encoding that does not conform to the spec is an old Outlook.
It is easy to say, "E-mail software not compliant with the spec is bad", but users of such e-mail software are never few. I think that we should not ignore this fact.

Regular users are not interested in the meaning of U + FFFD. They will simply decide that it is a bug in Thunderbird. And they will downgrade Thunderbird to version 52 or choose Microsoft products.

I hope that discussion will be conducted from the user's point of view.

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