CP866 is a very old standard, it was used in pre-Windows times. There are at least 3 more standards to encode cyrillics in 8 bits. Today, most Cyrillic letters on the web are encoded in either UTF-8 or CP1251.
All of them define the whole alphabete, though, so even the letters that look similar to some latin letters are always encoded differently.
All of them define the whole alphabete, though, so even the letters that look similar to some latin letters are always encoded differently.