XML in a Nutshell, 3rd Edition

Name

Comments

Synopsis

                        <!-- 
                        comment text 
                        -->

Comments can appear anywhere in a document or DTD, outside of other markup tags. XML parsers are not required to preserve contents of comment blocks, so they should be used only to store information that is not a part of your application. In reality, most information you might consider storing in a comment block probably should be made an official part of your XML application. Rather than storing data that will be read and acted on by an application in a comment, as is frequently done in HTML documents, you should store it within the element structure of the actual XML document. Enhancing the readability of a complex DTD or temporarily disabling blocks of markup are effective uses of comments.

The character sequence -- cannot be included within a comment block, except as part of the tag closing text. Because comments cannot be nested, commenting out a comment block is impossible.