HTML & XHTML: The Complete Reference (Osborne Complete Reference Series)

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 (DATAREFOBJECT) 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.