I see that many sites (amazon, wikipedia, others) use UTF8-encoded, URL-escaped unicode in their URLs, and those URLs are prettified by (at least) Chrome.
For example, we would represent http://ja.wikipedia.org/wiki/メインページ as http://ja.wikipedia.org/wiki/%E3%83%A1%E3%82%A4%E3%83%B3%E3%83%9A%E3%83%BC%E3%82%B8 when writing our http headers, and Chrome and Firefox seem to understand this in a graceful way. (I didn't test on IE.)
Is there a governing standard for this behavior? Or is it strictly a de facto standard? Or is it completely non-standard?
I'd really like to see a link to the defining paragraph of some RFC.