That ("if you're presented with that and don't send it to that host, you've violated the RFC") has nothing to do with the comment you responded to, which descri

That ("if you're presented with that and don't send it to that host, you've violated the RFC") has nothing to do with the comment you responded to, which described a UI choice compatible with RFC 3986, which says "Applications should not render as clear text any data after the first colon (":") character found within a userinfo subcomponent" (and which also goes on to say "Applications may choose to ignore or reject such data when it is received as part of a reference").