Jabber
From:  http://docs.jabber.org/draft-miller-impp-jabber-00.html

Appendix A. The Jabber Protocol DTD

<!--
    Jabber XML DTD
    By Julian "X-ViRGE" Missig
    julian@linuxpower.org
-->

<!--=========== The Main Elements =============-->

<!-- jabber, the root element -->
<!-- note: the root element is not normally used due to the use of xmlstreams and namespaces -->
<!ELEMENT jabber ((presence | iq | message)*)>

<!-- presence, a subelement of jabber -->

<!ELEMENT presence ((status? | priority? | show?)*)>
<!ATTLIST presence
  to          CDATA             #IMPLIED
  from        CDATA             #IMPLIED
  type        (subscribe | subscribed | unsubscribe | unsubscribed | unavailable | probe) #IMPLIED
  >

<!ELEMENT status (#PCDATA)>

<!ELEMENT priority (#PCDATA)>

<!ELEMENT show (#PCDATA)> <!-- standard options are: chat, away, xa and dnd -->


<!-- message, a subelement of jabber -->

<!ELEMENT message ((body? | error* | subject? | thread?)*)>
<!ATTLIST message
  to          CDATA             #IMPLIED
  from        CDATA             #IMPLIED
  id          ID
  type        (normal | error | chat | groupchat | headline) #IMPLIED
  >

<!ELEMENT body (#PCDATA)>

<!ELEMENT error (#PCDATA)>

<!ATTLIST error
  code        CDATA        #IMPLIED
  >

<!ELEMENT subject (#PCDATA)>

<!ELEMENT thread (#PCDATA)>

<!ELEMENT x (#PCDATA)>


<!-- iq, a subelement of jabber -->

<!ELEMENT iq (#PCDATA)>
<!ATTLIST iq
  to          CDATA        #IMPLIED
  from        CDATA        #IMPLIED
  id          ID
  type        (get | set | result | error)  #IMPLIED
  >