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

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.