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"4.2.5.4 Other pragma directives Extensions to the predefined set of pragma directives may, under certain conditions, be registered in the WHATWG Wiki PragmaExtensions page. [WHATWGWIKI] Such extensions must use a name that is identical to a previously-registered HTTP header defined in an RFC, and must have behavior identical to that described for the HTTP header." This goes further than the IETF requirements defined in RFC 3864. Recommendation: just state that the header needs to be registered with IANA; *potentially* requiring a registration in the permanent registry.
I updated this a while back.