The rule could be applied to a single HTML, HTML For this reason, while XML was originally designed for use as a more sophisticated document representation language to supplant HTML on the Web this particular aspect of XML (the creation and application of vocabularies) has been slow to take off, for a number of reasons: To date only internet Explorer 5 supports the use of XML within the browser but it does so inconsistently with the standards. To create a full document description language, you need to be able to break a document in abstract into multiple logical units this is often a more difficult undertaking when working with document than dealing with well-defined data.
There is typically more than one mapping of a document that’s appropriate for the situation and resolving what makes up the best such mapping can typically be a time-consuming and a frequently politically perilous occupation. Documents are more likely to have a requirement for a consistent display and because of the sheer scope of such documents this can mean creating highly complex display mechanisms. When XML itself was in development, most of the working group responsible for creating XML came from a background in SGML in SGML a technology that is highly geared toward document creation and management so it is neither surprising that much of the early focus on XML concentrated on XML document aspect (especially as a replacement for HTML) nor surprising at least in retrospect that this usage of XML has yet to really take off.
For the record document-based XML is usually referred to as being loosely structured; elements typically can be placed in nearly and order text can mix freely with elements within container elements and elements can usually have fairly complex sets of attributes with many of the attribute values defaulted. In SGML the relationship between the elements is defined in a separate document called a document type definition which XML also supports. However in the future DTDs will almost certainly be deprecated in favor of XML schemas. (See the “XML as a Data Format” section later in this chapter of details.)
Perhaps the best example to demonstrate a document-centric approach is XHTML which is an XML-complaint version of HTML. With XHTML you have the mixed benefits of being able to parse the structure as an XML document (scanning for heads and subheads for example) while still being able to read it in a Web browser. XML as a Document Management Format. I want to make a distinction here between document formats and document management formats. Document management formats are very closely allied to the other current rages in computing circle these days’ directories.
While the conventional example of a directory is something like a phone book, I personally have had more than my share of problems with this description. In computer circles a directory is essentially a hierarchical structure that can be queried to retrieve information about a computer system the file structure on a network drive, user information and so forth.