How to inform remote server

According to Hypertext Transfer Protocol -- HTTP/1.1 (RFC 2616) your browser may request a particular document character set using the Accept-Charset header field. For example,

 Accept-Charset: koi8-r, windows-1251; q=0.8 

means that your browser is aware about koi8-r and windows-1251 character sets besides the default iso-8859-1 that any browser must be able to handle. If no quality (q) parameter is given, the value of 1.0 is assumed (as for koi8-r character set in this example). Character sets with bigger quality values are preferred.

If no Accept-Charset field is given, any character set is acceptable. In this case you can't tell the server that you use KOI8-R character set and it can feed you with, say, CP1251.