bug#20623: XML and HTML files with encoding/charset="utf-8" declaration loose BOM; Coding system is reset from utf-8-with-signature to utf-8 on save

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: sledergerber@gmx.net,  20623@debbugs.gnu.org
> Date: Fri, 22 May 2015 17:51:07 -0400
> 
> >> > What would you expect Emacs to do instead?  It just obeys the stated
> >> > encoding, which says nothing about the BOM.  How can Emacs know when
> >> > to use utf-8 and when utf-8-with-signature?
> >> To the extent that Emacs has seen the BOM when opening the file, it
> >> would make sense for Emacs to try and preserve this detail.
> > It does.
> 
> While there are cases where it does, this bug report is about a case
> where it doesn't, IIUC.

AFAIU, that happened because the user has this in ~/.emacs:

  (setq-default buffer-file-coding-system 'utf-8-dos)

IMO, this bad customization should be removed, and then the problem
will go away.