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<http://my.opera.com/operawiki/forums/topic.dml?id=1132462> Opera's use of file://localhost/ instead of file:/// has been a major compatibility problem with NPAPI plug-ins and external programs. Given those problems and that no other browser uses 'localhost', using file://localhost/ should be invalid or heavily discouraged. The link mentions Opera bug CORE-40495, but CORE-5928 and CORE-12340 are a few more examples (but far from all). The only thing mentioning file://localhost/ is <http://tools.ietf.org/id/draft-hoffman-file-uri-03.txt>, so Opera probably gets its behavior from that. But, no other browser (and very few programs) use it, so the RFC should be ignored and no browser should use file://localhost/ when file:/// works fine.
The specification does not mention the weird localhost behavior which should be sufficient I think. Only implement that what is required. Everything else is forbidden.
Sounds good.