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For now, XHTML is assembled using the parameter entities and DTD modules just described. XHTML 1.1 - Module-based XHTML includes a driver file and a customization file that handle the formal integration, as well as a set of rules for XHTML 1.1 conformance. Much of the work done in XHTML 1.1 - Module-based XHTML builds on the work performed in the other drafts, incorporating them by reference without describing what's happening.
Note The XHTML profiles provided by the W3C include SGML Open Catalog entries in addition to FPIs, but it isn't clear from the drafts whether Open Catalog entries are required or optional parts of XHTML modularization projects. For more on how to build SGML Open Catalog entries, see http://www.oasisopen. org/html/a401.htm. The Formal Public Identifier is the easiest part to create. FPIs contain four fields, separated by doubleslashes (//). The first field is always a dash, unless the resource the FPI describes is created by a formal standards organization such as ISO. (If the FPI is created by a standards organization, this field identifies the standard to which it refers.) The second field contains the name of the organization maintaining the resource. For the XHTML specification, this is W3C. For your own FPIs, it should be an identifier for your organization – possibly a domain name. The third field contains a unique name, chosen within your organization, which describes the referenced resource and includes version information as well. The final field should contain a language identifier (from the list in Appendix C). The language identifier describes the language within the resource and doesn't place any limitations on the content of documents using it. For example, the FPI for XHTML 1.1 is this:
-//W3C//DTD XHTML 1.1//EN
The first field is a dash (-) because the W3C is technically a vendor consortium, not a standards body proper. The second field contains W3C, the identifier used by the World Wide Web Consortium (W3C). The third field identifies the resource as a Document Type Definition (DTD) for XHTML 1.1. If the DTD changes for version 1.2 or 2.0, the W3C can change the version number within the FPI. Finally, EN identifies that the descriptions used in the DTD are written in English. (If the W3C provides a French translation, it might be FR. But French-language documents can refer to the English DTD without any problem.) Creating your own FPIs is easy. First (unless you work for ISO), start with a dash. Then identify your organization, followed by your project, followed by the language in which the DTDs are written. For example, if I create a profile that describes "Simon's Markup Language" version 1.0, the FPI might look like this:
-//simonstl.com//DTD SimonML 1.0//EN
Profile creators also should support a system identifier, typically a URL, where you can retrieve the DTD or other resource. Then the combination of the public and system identifiers is used in the DOCTYPE declaration by documents using the profile:
<!DOCTYPE simonML PUBLIC "-//simonstl.com//DTD SimonML 1.0//EN" "http://www.simonstl.com/xhtml/chap15/simonML.dtd">
The system identifier should refer to the driver file for the XHTML profile, just as the identifiers for XHTML 1.1 and XHTML Basic refer to their driver files. Driver files contain comments describing the module, a few entity declarations, and then an entity declaration that references and includes the customization file. After the customization file is included, the rest of the modules that make up the DTD are included using the .mod and .module parameter entities just described.
XREF It isn't clear quite how the W3C expects these files to fit into Web infrastructure. While XML processors routinely load resources like DTDs from across the Internet, that approach hasn't been used for the most part with HTML processors. Most likely, the W3C expects processors to use the FPI in the DOCTYPE declaration to figure out what they have to work with, and then apply logic specific to that document type rather than generic XML validation and processing. Because DOCTYPE declarations require a URI in addition to the FPI, however, ordinary XML processors will be capable of processing documents using the information retrieved from that URI. SGML Open Catalog entries (also provided in the XHTML specifications) provide another way for generic XML processors to interpret the FPI and retrieve the needed information.
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