What's a Unix?
Unix98 and The X/Open Single UNIX Specification seem to be used interchangeably, but formally a Unix98 is an OS that conforms to The X/Open Single UNIX Specification, ver. 2. (drafts for ver. 3 are here.) See also here for some info about various Unix98 systems.
The specification is published by The Open Group. (site index, publications and standards, or search engine)
The Uniform Resource Identifiers (URI) Working Group of the Internet Engineering Task Force (IETF) is publishing Internet RFCs about the syntax of URIs and URLs.