x-metadata
x-metadata provides specific information about the book itself. In case of using Microsoft Reader, x-metadata can contains <meta name="ms-chaptertour"..> tag

Example: 

<metadata>
 <x-metadata>
  <meta name="ms-chaptertour" content="chaptertour"/>
 </x-metadata>
</metadata>

attributes

id
Provides an element with a unique name within a single document. This is especially useful in works with extensive cross-referencing and linking, such as dictionaries, encyclopedias, or travel guides. Use of ids  simplifies linking and updates.
class
Affiliates an element with one or more classes.
style
Attaches a list of inline (name:value paired) CSS properties to a specific tag, for example: style="name:value;name:value"

See also:

<meta>