When documents contain content and data about the content, the two types of data should be clearly separated.
A document contains two distinct types of data, the content of the document, and data about the content. The data about the content is referred to as metadata. Since a document contains these two different types of data, it is not always to distinguish between the two types of data. For example:
<Name>Patterns of Stereo Design</Name>
Use of patterns to arrange stereo components.
Here there are two instances of the Author element, and at first glance it might not be possible to tell what the first instance of Author represents. Is it the author of the article being summarized? Is it the author of the article itself? It can be difficult to distinguish metadata from data.
Data about the data needs to be included in a document. This could be things like the author's name, the creation date, security levels of the data, namespace information, schema information, or identification attributes for use with cross references.
A clear separation is needed between what is metadata and what is data that forms the body of the document. This affects ease of authoring and processing of the document because the context of the data is clear.
The context of the data and the metadata should be made clear. The metadata should usually appear before the data that it describes. This makes it clearer what the metadata is about, and allows processing software to know about the data before it actually gets the data. For example the size of a table might be considered metadata. If the processing software gets the size of the table before the actual data, it can layout the table and then insert t he data in the proper place as it encounters it.
The resulting context provides structures that clearly identify the metadata as metadata. Often this pattern introduces new constructs to the document, so the overall length of the document may be increased. Authors and processing software need to clearly distinguish between metadata and content. This is not always possible to tell from the element names or positions. It is better to provide a context that will disambiguate the types of data. Obviously the first step in using this pattern is to be able to identify the difference between metadata and data. This is not always an easy task.
Metadata in Separate Document
are specializations of this pattern.
W3C Namespace Recommendation includes namespace information in attributes that make it clearer that this is data about the documents, and not really part of the document itself.
The XHTML DTD uses Head and Body elements to distinguish the metadata from the data.