DTDs in both SGML and XML contain the type of comments familiar to Web page authors:
<!-- this is a comment -->
Comments can also be embedded inside SGML declarations for explanatory purposes. Embedded comments are delimited by two dashes, and a single declaration may contain many embedded comments.
<!ATTLIST PARAM name CDATA #REQUIRED -- property name -- value CDATA #IMPLIED -- property value -- valuetype (DATA|REF|OBJECT) DATA -- How to interpret value -- type CDATA #IMPLIED -- Internet media type -- >
However, XML does not use this comment style, so you will not see it in the XHTML specification.