Custom headers do not REQUIRE a prefix. So the lack of prefix does not make it non-compliant. In fact, the HTTP RFC doesn't even say they SHOULD have a prefix. And RFC822 which defines headers only says that protocol mandated headers MUST NOT be prefixed with "X-". Also, there's a draft proposal to deprecate the "X-" header prefix as it does more harm than good: http://tools.ietf.org/html/draft-ietf-appsawg-xdash-02

Second, absolute URIs are perfectly valid, all HTTP/1.1 servers MUST accept them: http://www.w3.org/Protocols/rfc2616/rfc2616-sec5.html#sec5.1