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I'm amazed no-one's mentioned XMPP/Jabber.
Last time I read the XMPP they hard-code the values of the namespace prefix. That is to say that every XMPP server and client *must* use the same value for the prefix. The URI is irrelevant.
It's not real XML namespace use, but it does seem to work for tens of millions of users on a daily basis.
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