"X-" Prefixes deprecated by RFC 6648.
I would like to share with you the recently published RFC 6648, which
deprecates the use of "X-" prefixes in "application protocols"
BEST CURRENT PRACTICE
Internet Engineering Task Force (IETF) P. Saint-Andre
Request for Comments: 6648 Cisco Systems, Inc.
BCP: 178 D. Crocker
Category: Best Current Practice Brandenburg InternetWorking
ISSN: 2070-1721 M. Nottingham
Deprecating the "X-" Prefix and Similar Constructs
in Application Protocols
Historically, designers and implementers of application protocols
have often distinguished between standardized and unstandardized
parameters by prefixing the names of unstandardized parameters with
the string "X-" or similar constructs. In practice, that convention
causes more problems than it solves. Therefore, this document
deprecates the convention for newly defined parameters with textual
(as opposed to numerical) names in application protocols.
Full text available at http://tools.ietf.org/html/rfc6648
Tsurumi, Kanagawa, Japan