UTF-16

Yes, sorry, of course wikitext is markup. But it's (by design) very lightweight markup that only accounts for a small fraction of the text in most cases. If you're using something like HTML, let alone a programming language, UTF-8 is a clear win for East Asian text. tetromino's data suggests that even in a case with a huge advantage for UTF-16 (CJK with only light markup), it's still only a few percent smaller. That's just not worth it, when UTF-8 is *much* smaller in so many real-world cases.