As I mentioned, I first ran into this problem on the Emacs Wiki (with the
same em-dash character, in a library that is derived from buff-menu.el).
Simply uploading or downloading the code on the Wiki changes the characters
(in the same way, BTW). Here, the downloading user has no choice. If the
normal page-edit means of uploading is used, then the characters are messed
up in the file on the wiki, so regardless of how you download it, you get
garbage. AFAIK, this has nothing to do with the browser. You might not care
about the Emacs Wiki, but you might care that such a problem exists there,
because other sites might present similar problems.