2.6. Miscellaneous Markup
Elements, attributes, namespaces, and entities are the most important markup objects, but they are not the end of the story. Other markup objects including comments, processing instructions, and CDATA sections shield content from the parser in various ways, allowing you to include specialized information.
Comments are notes in the document that are not interpreted by the parser. If you're working with other people on the same files, these messages can be invaluable. They can be used to identify the purpose of files and sections to help navigate a cluttered document, or simply to communicate with each other. So, in XML there is a special kind of markup called a comment. The syntax for comments is shown in Figure 2.17.
Figure 2.17. Syntax for comments
A comment starts with four characters: an open angle bracket, an exclamation point, and two dashes (1). It ends with two dashes and a closing angle bracket (3). In between these delimiters goes the content to be ignored (2). The comment can contain almost any kind of text you want, including spaces, newlines, and markup. However, since two dashes in a row (--) are used tell the parser when a comment begins and ends, they can't be placed anywhere inside the comment. This means that instead of using dashes to create an easily visible line, you should use another symbol like an equals sign ...