Perhaps an HTML+ tag
<!ELEMENT EXTENSION - - #PCDATA -- extensions > <!ATTLIST EXTENSION id
which is used in the HEAD as follows
<EXTENSION id="ncsa-anicon">NCSA ANICON tags to allow animated icons
in the text</EXTENSION>
Used basically as a way of describing and informally naming any
extensions to the DTD. Browsers ignore them and their contents.
Whenever an extension is implemented, the DTD extension should be sent
to the mailing list and the extension discussed.