[1] 
[[BEEP]] ([[RFC 3080]]) は、 [[IANA]]
に登録''しない''[[機能識別]]が [CODE@en[[[x-]]]]
で始まることを求めています。
しかし、 [[IANA]] が [CODE@en[[[x-]]]]
で始まる[[文字列]]を登録してはいけないとは述べられていません。
[[大文字]]と[[小文字]]の区別には言及がありません。

[2]
[[vCard]] [CODE@en[[[IMPP]]]] [[型]] ([[RFC 4770]])
は、 [CODE@en[[[TYPE]]]] [[引数]]の値の一部として
[CODE(ABNF)@en[[[x-name]]]] の使用を認めています。
[CODE(ABNF)@en[[[x-name]]]] の定義は [[RFC 2426]]
にあって、非標準な利用のためのものとされています。

[3]
[CITE@en-us[XLIFF 1.1 Specification]] ([CODE[2006-07-08 02:30:08 +09:00]] 版) <http://www.oasis-open.org/committees/xliff/documents/cs-xliff-core-1.1-20031031.htm#Struct_Extension>
([[名無しさん]] [sage])

[4]
[CITE@en-us[XLIFF 1.1 Specification]] ([CODE[2006-07-08 02:30:08 +09:00]] 版) <http://www.oasis-open.org/committees/xliff/documents/cs-xliff-core-1.1-20031031.htm#Struct_Extension_Values>
([[名無しさん]] [sage])

[5]
[CITE@en-us[XLIFF 1.1 Specification]] ([CODE[2006-07-08 02:30:08 +09:00]] 版) <http://www.oasis-open.org/committees/xliff/documents/cs-xliff-core-1.1-20031031.htm#mtype>

[17] [CITE[XLIFF 1.2 Specification]] ([TIME[2008-02-05 00:30:42 +09:00]] 版) <http://docs.oasis-open.org/xliff/v1.2/os/xliff-core.html#Struct_Extension>

[6]
[CITE[Global Information Management eXchange Metrics Volume 1.0 Specification]] ([CODE[2007-02-24 17:44:10 +09:00]] 版) <http://www.lisa.org/standards/gmx/GMX-V.html#CountExtension>
([[名無しさん]] [sage])

[7]
[CITE[Global Information Management eXchange Metrics Volume 1.0 Specification]] ([CODE[2007-02-24 17:44:10 +09:00]] 版) <http://www.lisa.org/standards/gmx/GMX-V.html#Attr_state>
([[名無しさん]] [sage])

[8]
[CITE@en-us[TMX Specification]] ([CODE[2007-02-24 17:51:20 +09:00]] 版) <http://www.lisa.org/standards/tmx/tmx.html#prop>
([[名無しさん]] [sage])

[9]
[CITE@en[RFC 4408 Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1]] ([CODE[2007-02-14 19:02:25 +09:00]] 版) <http://tools.ietf.org/html/rfc4408#section-7>

[11] [CITE[SMIL 3.0 Content Control]] ([TIME[2008-12-02 00:45:09 +09:00]] 版) <http://www.w3.org/TR/2008/REC-SMIL3-20081201/smil-content.html#adef-systemBaseProfile>

[10] [CITE[SMIL 3.0 Structure Module]] ([TIME[2008-12-02 00:45:09 +09:00]] 版) <http://www.w3.org/TR/2008/REC-SMIL3-20081201/smil-structure.html#adef-baseProfile>

* 他のもの

[12] [[名前空間]]を使うもの (e.g. [[XML]] 系[[マーク付け言語]]、[[XPointer scheme]])

[13] [CITE[JSON-RPC 1.1 Working Draft 7 August 2006]] ([TIME[2007-01-11 07:38:32 +09:00]] 版) <http://json-rpc.org/wd/JSON-RPC-1-1-WD-20060807.html#ObjectExtensions>

[CODE(char)[[[$]]]] で始まるもの。

[14] [[vnd.*]]、[[prs.*]]: [[媒体型]]

[15] [[-[VAR[vendor]]-*]]: [[CSS]]、[[選択子]]

[16] [[[[vendor]]*]]、[[[VAR[Vendor]]*]]: [[DOM]]


[18] [CITE[IRC logs: freenode / #whatwg / 20090918]]
([TIME[2009-11-03 01:07:53 +09:00]] 版)
<http://krijnhoetmer.nl/irc-logs/whatwg/20090918>

[19] [CITE@@en[Web Applications 1.0 r5307     use vendor--feature instead of _vendor-feature since Apple engineers think underscores are ugly.Fixing http://www.w3.org/Bugs/Public/show_bug.cgi?id=9590]]
( ([TIME[2010-08-17 07:14:00 +09:00]] 版))
<http://html5.org/tools/web-apps-tracker?from=5306&to=5307>

[20] [CITE@en[Speech Synthesis Markup Language (SSML) Version 1.1]]
( ([TIME[2010-09-08 00:26:19 +09:00]] 版))
<http://www.w3.org/TR/2010/REC-speech-synthesis11-20100907/#edef_phoneme>

[21] [CITE@en[Web Applications 1.0 r5562 Change how vendor extensions are marked up.Fixing http://www.w3.org/Bugs/Public/show_bug.cgi?id=9590]]
( ([TIME[2010-09-30 07:38:00 +09:00]] 版))
<http://html5.org/tools/web-apps-tracker?from=5561&to=5562>

[22] [CITE[IRC logs: freenode / #whatwg / 20100930]]
( ([TIME[2010-10-11 01:27:16 +09:00]] 版))
<http://krijnhoetmer.nl/irc-logs/whatwg/20100930>