David Baron in Why Web authors must specify character encodings: Bad Web content often doesn't tell the browser which
encoding to use, so Web browsers often guess based on the user’s
language. So an American’s Web browser would guess the normal encoding
used for English, but a Japanese user's browser would guess the normal
encoding used for Japanese. This means that even if a page works
fine for you and all your colleagues, it might not work for somebody in
Japan. Or for me, because I have my browser configured strangely.
And also: If everybody used UTF-8, all these problems could go away.
See also the addendum on the unification of CJK characters.