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Reportedly Internet Explorer supports shift_jis as part of its iso-2022-jp decoder. We should probably do the same as (again reportedly) sites rely on that. And if sites rely on it, supporting it would be better than requiring detection.
It certainly is the case that IE's 7-bit CJK decoders incorporate an 8-bit decoder for the same language, or at least did so in the past. (I thought I had reported that at some point.)
The mail has more dependency than the site. I receive mail messages with mislabeled subjects everyday.
*** This bug has been marked as a duplicate of bug 16685 ***