Modularization of XHTML^™ in XML Schema

W3C Working Draft 19 December 2001

This version:

http://www.w3.org/TR/2001/WD-xhtml-m12n-schema-20011219
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Latest version:

http://www.w3.org/TR/xhtml-m12n-schema

Previous version:

http://www.w3.org/TR/2001/WD-xhtml-m12n-schema-20010322/

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xhtml-m12n-schema-diff.html

Editors:

Daniel Austin, Invited Expert
Shane McCarron, Applied Testing and Technology

Copyright ©2001 W3C^® (MIT, INRIA, Keio), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply.

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Abstract

This document describes a methodology for the modularization of XHTML using XML Schema. Modularization of XHTML allows document authors to modify and extend XHTML in a conformant way.

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. The latest status of this document series is maintained at the W3C.

This is an internal draft for review by the HTML Working Group and other interested parties. It is a stand-alone document to ease its review. Once the methodology described in this document become mature, it will be integrated into a future document forthcoming from the HTML Working Group.

This document is still in its early stage, and may be updated, replaced, or obsoleted by other documents at any time. Publication of this Working Draft does not imply endorsement by the W3C, and it is inappropriate to use W3C Working Drafts as reference material or to cite them as other than "work in progress". A list of current W3C Recommendations and other technical documents can be found at http://www.w3.org/TR.

Please send review comments to www-html-editor@w3.org (archive). Public discussion on HTML features takes place on the mailing list www-html@w3.org (archive).

This document has been produced by the W3C HTML Working Group (members only) as part of the HTML Activity. The goals of the HTML Working Group are discussed in the HTML Working Group charter. The W3C staff contact for work on HTML is Masayasu Ishikawa.

Quick Table of Contents

Full Table of Contents

1. Introduction

This section is informative.

1.1. Purpose of this document

The purpose of this document is to describe a modularization framework for languages within the XHTML Namespace using XML Schema [XMLSCHEMA]. There are currently several public language variants in the XHTML namespace, including XHTML 1.0 [XHTML10] (which includes variants corresponding to the definitions of "strict", "transitional", and "frameset") and XHTML Basic [XHTMLBASIC]. The development of DTD-based modularization for XHTML made it possible to refashion XHTML 1.0 in a modularized way [XHTMLMOD], resulting in XHTML Basic and XHTML 1.1 [XHTML11]. (Here and throughout this document, the term "XHTML-MOD" is used to refer to [XHTMLMOD].)

This document provides a complete set of XML Schema modules for XHTML. In addition to the schema modules themselves, the framework presented here describes a means of further extending and modifying XHTML.

To the largest extent possible, the modularization framework presented here attempts to duplicate the modularization concepts used in XHTML-MOD. Data structures in the modularized DTDs are in many cases mapped directly onto data structures in XML Schema. This method does not yet however, make extensive use of XML Schema-specific features.

This document is based on an approach to modular schemas originally suggested by Rick Jelliffe and members of the XML Schema Working Group at W3C. [APPROACH]

1.2. Why Modularize?

In the development of any type of complex application, it is important to follow a clear conceptual standard for organizing the development. The modular approach to design reduces the application's functionality into some number of "building blocks" or "modules". These modules are then combined according to specific rules to form the entire application. This approach offers numerous advantages:

• Conceptual clarity allows developers to share ideas and code
• Reduces complexity by decomposition of the application's functionality
• Supports object-oriented design principles by encouraging encapsulation and information hiding
• Encourages reuse by creating well-defined modules that perform a particular task
• Decreases debugging time by localizing errors due to design changes
• Increases flexibility and maintainability because single modules can be upgraded or replaced independently of others
• Eases development, testing, and maintenance by providing a logical, easy to understand, and consistent organization
• Allows the creation of generic rules, methods, and procedures to aid in consistent development practices
• Creates configurable objects that the end user can tailor for different purposes
• Supports a variety of end user interface and deployment environments by allowing standardized subsets and supersets.

1.3. Design Goals

These are the design goals for this modularization framework for XHTML:

• To create coherent sets of semantically related modules within the XHTML namespace using XML Schema
• To support the creation of subsets and supersets of XHTML for specific purposes such as handheld devices and special-purpose appliances
• To facilitate future development by allowing modules to be upgraded or replaced independently of other modules
• To encourage and facilitate the reuse of common modules by developers.

1.4. Requirements

This document describes a modularization framework that attempts to reuse the conceptual ideas in XHTML-MOD, but does not attempt to literally duplicate them in all aspects.

The DTD modularization framework described in XHTML-MOD is subject to a detailed and explicit list of requirements [XHTMLMOD]. The scope of the schema-based framework described here is also constrained by this set of requirements, and is believed to have fulfilled them in their entirety.

2. Schema Modularization Framework

This section is informative.

2.1. How Schema Modularization Works

2.1.1. DTDs and XML Schema

Both DTDs and XML Schema are designed to accomplish the same fundamental task: to define the structure of XML document types. In this sense both are simply different text representations for the same underlying data structures. However, Schema and DTDs differ significantly in several ways, both in structure and capabilities.

Some differences worth noting are:

Common XML features

XML Schema are XML documents themselves and therefore share many aspects of the languages they define.

Data typing

Schemas are designed with a much larger set of built-in data types than DTDs, and provide methods for creating user-defined types.

Namespaces

DTDs only partially support XML Namespaces, which are inherently a part of XML Schema.

Extension

XML Schema have a rich set of extension mechanisms including inheritance, redefinition, and substitution.

Entities

There is no mechanism in XML Schema corresponding to the use of entities for data abstraction in DTDs. In many cases the functionality of entities can be replaced through other XML-based mechanisms. However, there is currently no support for named character entity references as used in XHTML within XML Schema.

DTDs and Document Order Dependence

A more subtle feature of modularized DTDs is their dependence on the document order; the order in which elements and entities are defined within DTD files has a large impact on language development. XML Schema are far less dependent on document order.

2.1.2. Document Data Structures

XML language definitions, regardless of their text representation, contain at least three types of data structures. When combined into a coherent and consistent whole, they form a complete language definition. These three components are:

• Elements
• Attributes
• Content models

Additional abstract data structures may be defined for use in the language definition, such as common content models or attribute groups, whose use is shared by other data structures within the language definition. The definition of these structures is the primary task of language development, and the core of the modularization framework.

2.1.3. Understanding XHTML Modularization

This schema modularization framework consists of two parts:

1. A set of schema modules that conform to the abstract modules in XHTML
2. A set of modularization conventions that describe how the individual modules work together, and how they can be modified or extended.

In XHTML-MOD, every object in the DTDs is represented by an XML entity. These entities are then composed into larger sets of entities and so on, resulting in a set of data abstractions that can be generalized and used modularly. These multiple levels of abstraction are tied together by the use of a specific naming convention and a set of abstract modules.

Generic classes of entities (composed of sub- and sub-sub-entities) are used to create definitions of the three components listed above. Content models, attribute lists and elements are defined separately, sometimes in separate modules, and the ordering of the modules in the DTD structure is strictly defined (due to document order dependence). They are then combined to form the resulting document type. Extensibility is accomplished through the extensive use of INCLUDE/IGNORE sections in the DTD modules. How each of these structures relates to its Schema-based counterpart is summarized in Table 1 below.

2.1.4. Mapping DTDs onto Schema

Both the DTD and schema-based modularization frameworks implement a set of formalized data structures, often in a conceptually similar way. The modularization framework described here is designed around the use of similar data structures, which can be represented (more or less) equally well in either representation. This is accomplished through the use of a straightforward mapping of data structures defined in the DTD modules onto equivalent data structures in the XML Schema language.

2.1.4.1. Content Models

In XHTML-MOD, content models for elements are defined using three classes of entities, identified through the naming conventions by the suffixes ".content", ".class", and ".mix". Each of these classes of entities is mapped onto a corresponding Schema counterpart in the following way:

".content" models   - these models are used to define the contents of individual elements. For each element there is a corresponding ".content" object. IN XML Schema, ".content" entities are mapped directly onto groups:

<ENTITY % html.content "(head+,body+)">

<group name="html.content">
   <sequence>
      <element ref="head" minOccurs="1">
      <element ref="body" minOccurs="1">
   </sequence>
</group> 

The contents of ".content" groups are often classes or mixes.

".class" models - these models are used to define abstract classes of content models made up of either ".content" entities or other ".class" entities (or elements). In XML Schema they correspond to groups that may also contain substitution groups:

<!ENTITY % Misc.class "%Edit.class;
                 %Script.class;
                 %Misc.extra;"> 

<group name="Misc.class">
   <choice minOccurs="0"
           maxOccurs="unbounded">
      <element ref="Edit.class"
               abstract="true"/>
      <element ref="Script.class"
               abstract="true"/>
      <element ref="Misc.extra"
               abstract="true"/>
   </choice>
</group>

".mix" models - these models correspond to content models that are mixed groupings of  ".class", ".content", and ".mix" entities and serve as abstract content models often used in common by many elements in the DTD. They correspond to groups in XML Schema:

<!ENTITY % Block.mix "%Heading.class;
                       | %List.class;
                       | %Block.class;
                       %Misc.class;">

<group name="Block.mix">
   <choice minOccurs="0"
           maxOccurs="unbounded">
       <group ref="List.class"/>
       <group ref="Block.class"/>
       <group ref="Misc.class"/>
       </choice>
</group>

In addition to these three content model groupings, XHTML-MOD includes an additional grouping ".extra". These are currently omitted from the schema modules. (If needed, a developer could add them to the schema modules in a conformant way.)

2.1.4.2. Attributes and Attribute Groups

Attributes and Attribute lists in DTDs correspond directly to attribute and attributeGroup elements in XML Schema. The translation from one to the other is relatively simple and straightforward. Here is an example:

<!ENTITY % title.attrib
"title %Text.datatype; #IMPLIED">

<attributeGroup name="title">
   <attribute name="title" type="string"/>
</attributeGroup>

Complex attribute groups that are used by many different elements are grouped in the DTDs using entities suffixed with ".attrib". These attribute entities map directly onto attributeGroup elements in XML Schema as shown above.

2.1.4.3. Complex Types and Element Definitions

The XML Schema specification allows elements as well as attribute values to be strongly typed. In defining elements in the modularized schema, an element type is created for each element that is a complex type composed of the content model (element.content) and the attribute list (element.attlist) as shown below:

<complexType name="form.type">
    <group ref="form.content"/>
    <attributeGroup ref="form.attlist"/>
</complexType>

Elements are then declared to be of the type element.type:

<element name="form" type="form.type"/>

This allows the author the greatest degree of flexibility while retaining strict type checking via XML Schema. It also allows for extension of the element via type substitution.

Note that in the case of an element with a mixed content model, a complexType is necessary.

In summary, each element is composed of a content model and an attribute list, which are composed into a type for that element.

2.1.4.4. Attribute and Element Redefinitions

XML Schema allows inheritance and redefinition of elements, groups, attributes and attributeGroups. In several cases modules require modification of previously declared attribute lists. This is done by using the <xsd:redefine> element to redefine the attributeGroup that needs to be modified

<!- - new attribute to be added - - >
<attributeGroup name="align.legacy.attlist">
   <attribute name="align">
      <simpleType>
         <restriction base="NMTOKEN">
            <enumeration value="left"/>
            <enumeration value="center"/>
            <enumeration value="right"/>
            <enumeration value="justify"/>
         </restriction>
      </simpleType>
   </attribute>
</attributeGroup>

<!- - add it to the caption element's attribute group - - >
<redefine schemaLocation="xhtml-table-01.xsd">
   <attributeGroup name="caption.attlist">
      <extension base="align.attlist"/>
      <attributeGroup ref="align.legacy.attlist"/>
      </extension>
   </attributeGroup>
</redefine>

In this example, we redefine the attribute list for the caption element in the tables module to add the align attribute defined in align.legacy.attlist.

2.1.4.5. Support Structures

The modularized DTDs contain support mechanisms for XHTML. Some of these are DTD-specific and are not fully supported in XML Schema.

This modularization framework attempts to recreate these support structures to the greatest extent possible.

2.1.4.5.1. Notations

Notations are an SGML feature that allows non-SGML data within documents to be interpreted locally [CATALOG]. Notations for XHTML are preserved in the Schema modules using the notation element in a straightforward way.

<!NOTATION character
   PUBLIC "-//W3C//NOTATION
   XHTML Datatype: Character//EN">

<notation name="charset"
   public="-//W3C//NOTATION
   XHTML Datatype: Charset//EN"/>

2.1.4.5.2. Data Types

The strong typing mechanism in XML Schema, along with the large set of intrinsic types and the ability to create user-defined types, provides for a high level of type safety in instance documents. This feature can be used to express more strict data type constraints, such as those of attribute values, when using XML Schema for validation.

<!ENTITY % Length.datatype "CDATA" >

<simpleType name="Length">
   <restriction base="string"/>
</simpleType>

2.1.4.5.3. Named Character Entities

XML Schema provides no means of duplicating XHTML's named character entity mechanism. In most cases data abstraction through entities can be dispensed with in schemas. However, in the case of named character references, no replacement method is available.

Character entities are used to represent characters that occur in document data that may not be processed natively on the user's machine, for instance the copyright symbol. XHTML makes use of 3 sets of named character entities: the ISO Latin 1, Symbols, and Special.

A general solution for the resolution of language-specific named character entities is outside the scope of this document.

Entities are currently referenced in this framework as notations; this is strictly incorrect. However it is considered desirable to include references to the XHTML-specific entity sets.

Here is an example of the use of the notation declaration syntax for named character references:

<notation name="XHTML-Latin-1" 
   id="xhtml-lat1"
   public="-//W3C//ENTITIES Latin 1 for XHTML//EN" 
   system=" http://www.w3.org/TR/xhtml1/DTD/xhtml-lat1.ent"/>

2.1.4.6. Mapping Summary

The following table summarizes the mapping of DTD data structures onto XML Schema structures.

One further issue of note in the conversion of DTDs to XML Schema is that it is absolutely necessary to define all elements globally. Otherwise they are not considered to be in the XHTML namespace but only "associated"[XMLSCHEMA_COMPOSITION] with it. This document does not make use of this association feature in XML Schema.

2.2. Framework Conventions

This section is normative.

This modularization framework consists of a complete set of XHTML schema modules and a set of framework conventions that describe how to use them. The use of the framework conventions is required for conformance.

2.2.1. Modularized Schemas

The modularized XHTML schema uses three types of modules, which when combined comprise the entire XHTML definition.

2.2.1.1. Hub document

The Schema hub document is the base document for the schema. It contains only annotations and modules, which in turn contain <xsd:include> statements referencing other modules. The hub document corresponds to the DTD "driver" module in XHTML-MOD, but is much simpler. The hub document allows the author to modify the schema's contents by the simple expedient of commenting out modules that are not used. Note that some modules are always required in order to ensure conformance.

The (non-normative) example hub document described here contains <include> elements for two modules, named "required" and "optional". Each of these included modules is itself a container module.

2.2.1.2. Container Modules

Module containers, reasonably enough, include other modules. Modules and their containers are organized according to function. Including the hub document, which is a special case of a module container, there are ten included module containers.

2.2.1.3. Element modules

In addition to the module containers listed above, there are around forty schema modules which contain only element definitions and their associated attribute and content model definitions. By convention, Schema modularizations may contain either <include> statements or element definitions but not both.

2.2.2. Module Naming

In order to easily identify the contents of any particular schema module, it is useful to provide here a module naming convention syntax. This syntax also provides a simple means of distinguishing modules based on their language version, which may improve maintainability of the modules themselves.

The module naming convention adopted here is the same in almost all respects as that used in XHTML-MOD.

Schema modules for XHTML should have names that:

• Are supported on all common platforms
• Identify the contents of the modules
• Identify the language version of the module

Modules used in this modularization framework must have names that conform to the following syntax:

languagename-filecontentsdescription-versionnumber.xsd

xhtml-table-01.xsd

Exceptions to this rule are made for the Schema hub modules whose names are the same as above but may omit the content description syllable for brevity.

Version numbers of hub modules may omit the leading zero in the version number, but should include the minor version number.

Example: xhtml-1.1.xsd

In the case where a hub module contains elements or attributes from external namespaces, the name(s) of the external module(s) should be appended to the base language name using the "+" character.

Example: xhtml+fml-1.0.xsd

This module naming convention is intended also to comply with the required use of the media type in [XHTMLMIME].

2.2.3. Module Hierarchy Structure

In order to establish a physical structure for the composition of the Schema modules that corresponds to the abstract modules in XHTML, a module hierarchy structure has been used to organize the physical modules. The hierarchy structure looks like this:

xhtml/
xhtml/req/
xhtml/req/framework/
xhtml/req/core/
xhtml/req/core/text/
xhtml/opt/
xhtml/opt/pres/
xhtml/opt/legacy/
xhtml/opt/legacy/misc/
xhtml/opt/legacy/frames/

These correspond to the divisions of XHTML into abstract modules described in detail in Section 3.2. The hierarchy structure is intended to match the abstract module structure as closely as possible. This feature is not present in DTD modularization, and is not required for Schema modularization. It does, however, allow the developer to organize the modules in accordance with their hierarchical structure. The directories listed in Table 2 also correspond exactly to the module container modules in this framework.

2.2.4. Names for Data Structures

The consistent use of naming conventions is important for the maintenance and development of complex software applications. 

Adhering to these conventions provides numerous benefits to developers:

• Simplifies testing and debugging by managing complexity.
• Eases maintenance by allowing any developer to read and understand another developer's code.
• Provides self-documenting code by using descriptive names and predictable naming conventions.
• Enforces encapsulation by using consistent naming conventions for public and private knowledge.

With few exceptions, the naming conventions used in XHTML-MOD are preserved in this framework.

The naming convention in XHTML-MOD uses suffixing of object names to indicate functionality, as described below.

2.2.4.1. Attributes

Abstract attribute groups and attribute lists are suffixed with the ".attrib" and ".attlist" suffixes respectively.

2.2.4.2. Content models

Three different suffixes are used in content model names. They are ".content" for element content models, and ".class" or ".mix" for abstract content models.

2.2.4.3. Elements

Element names are not suffixed in XHTML-MOD. This document uses the notion of element types, which are complexTypes used to define elements and are suffixed with ".type". The ".type" suffix was used in XHTML-MOD for attribute data types. This is superfluous in XML Schema (since attribute types are arguments to the "type" attribute) and so the suffix is used in a different way in this framework.

2.2.5. Module Structure

This document establishes a convention for the internal structure of XHTML Schema modules. This convention provides a consistent and predictable way of organizing schema modules internally. This convention applies also to the hub document, which is itself simply a module of modules, albeit a somewhat specialized one.

Each schema module is composed of several components, some of which are required for functional reasons and some of which provide metadata as a convenience to the author. Not every component is included in every module.

2.2.5.1. Schema Element

Each module begins with a <xsd:schema> root element (after the optional xml declaration and DOCTYPE).

2.2.5.1.1. Use of Version Attribute

In the XHTML schema modules, the version number for the specific language being defined (e.g. "1.1") is used as the default value of the version attribute on the schema element.

2.2.5.1.2. Qualified names

This framework uses the value of "unqualified" for the value of the elementFormDefault attribute on the schema root element. Elements within the html namespace do not need to use a namespace prefix.

2.2.5.2. Annotation Block

After the root element each module contains an annotation element containing several documentation sections briefly describing the purpose of the module.

2.2.5.2.1. Module Description

This is an annotation element that contains a short description of the module and its purpose.

2.2.5.2.2. Versioning Block

An annotation element containing authoring and versioning information for the module should always be included.

2.2.5.2.3. Copyright

The standard W3C copyright statement is included in each module through the use of an include element. An exception is the hub document, which contains the full copyright text.

2.2.5.2.4. Documentation

This is a module specific documentation element providing detailed information about the module's contents, its organization, and any noteworthy items of interest to developers.

2.2.5.3. 3. Module elements

Module elements contain include statements, import statements, or other modules (or comments). They must precede any other definitions in the module.

2.2.5.4. 4. Content model groups

These include groups with names ending in ".content", ".class", or ".mix".

2.2.5.5. 5. Attributes and Attribute groups

These are suffixed with either ".attrib" or ".attlist".

2.2.5.6. 6. Element type definitions

These are complexType elements defining each element's type.

2.2.5.7. 7. Element definitions

These define individual elements in the module.

Additional constraints on the internal structure of schema modules are:

Each module must contain include statements for other modules or data structure definitions, but not both.

Each module must include at least sections 1 and 2 above, as well either section 3 or some combination of sections 4-7.

2.2.6. Namespace Conventions

The handling of namespaces in XML Schema is entirely different from that in XHTML-MOD. Namespaces are integral to XML Schema and their use in modularization arises naturally from the schema syntax.

One convention chosen for this framework is that the names of elements and attributes in the modules are unqualified i.e. no namespace prefix is required for XHTML elements.

This is set by using the value of "unqualified" on the elementFormDefault attribute of the xsd:schema element.

2.2.7. Documentation Conventions

A consistent commenting convention has been imposed on the modules described here. The purpose of a commenting convention is to allow for generating documentation from the comments (as well as general comprehension). Documentation elements containing Annotation-level comments are assumed to be of the highest importance and should be used to denote information about the module itself, and for important notes for developers.

ModuleF-level comments are denoted as usual with SGML comment delimiters "<!--" and "-->". By means of this convention, modules can become self-documenting. Tools for extracting these comments and formatting them suitably may (hopefully) be developed in the future.

3. XHTML Schema Modules

This chapter is normative.

3.1. XHTML Abstract Modules

The DTD modularization framework specification speaks at length on the subject of abstract modules. In brief, an "abstract" module is simply a set of objects, in this case objects within an ordered hierarchy of content objects, which encapsulates all of the features of the objects and assembles them into a coherent set. This set of objects and their properties is independent of its machine representation, and so is the same whether written in DTD module form, as a Schema module, or as a Java class.

The abstract modules described in XHTML-MOD are composed in a functional manner, and each "abstract module" contains data structures that are generally functionally similar. (There is no requirement that modules be created along functional lines; any other method that suits the author's purpose may be used instead.)

The framework described here makes use of the same abstract modules as in XHTML-MOD with few exceptions. In the case of the schema module representation, the relationship between the "abstract" modules and the schema modules is quite close. In each case there is a one-to-one relationship between the abstract and concrete modules (with one exception for the changes to the legacy module) and they share essentially the same names and data structures.

3.2. XHTML Schema Modules

3.2.1. Required Modules

These modules must be included in any document that uses the XHTML namespace. Each section below describes the purpose of the module and its contents.

None of the modules defined here should be modified by developers; instead use <redefine> or a substitution group.

3.2.1.1. Framework Modules

This is a module container for XHTML language support modules.

3.2.1.1.1. Notations

3.2.1.1.2. Data types

3.2.1.1.3. Common Attributes

3.2.1.1.4. Common Content Models

3.2.1.1.5. Character Entities

The character entities module includes three notation elements within an <appinfo> element, each referencing one of the required entity sets in XHTML: ISO Latin-1, Symbols, and Special characters.

Character entities are not fully supported in XML Schema, as described in Section 2.1.

3.2.1.2. Core Element Modules

These are the core element definitions for the required modules.

3.2.1.2.1. Text Modules

Block Phrasal

Block Structural

Inline Phrasal

  Inline Structural

3.2.1.2.2. Hypertext

3.2.1.2.3. Lists

3.2.1.2.4. Structural

3.2.2. Optional Modules

These modules are (clearly) optional; they may be removed or combined arbitrarily (except for dependencies). Developers should not modify the contents of these files as they part of the XHTML definition. Instead, extension in the optional modules should be confined to redefinitions and derivations.

3.2.2.1. Edit

3.2.2.2. Bdo

3.2.2.3. Presentational

3.2.2.3.1. Inline Presentational

3.2.2.3.2. Block Presentational

3.2.2.4. Link

3.2.2.5. Meta

3.2.2.6. Base

3.2.2.7. Scripting

3.2.2.8. Style

3.2.2.9. Image

3.2.2.10. Client-side Image Maps

3.2.2.11. Server-side Image Maps

3.2.2.12. Param

3.2.2.13. Applet

3.2.2.14. Object

3.2.2.15. Tables

3.2.2.16. Forms

3.2.2.17. Nameident

3.2.2.18. Legacy

This module has been reorganized to conform to the framework conventions used here. It has been divided here into two separate modules. The "misc" module contains everything in the DTD legacy model except frames. Frames are now in a separate module called framedefs. This allows the developer to easily separate the legacy features if desired.

3.2.2.18.1. Misc

3.2.2.18.2. Framedefs

Frames

Target

Iframe

3.2.2.19. Basic Forms

3.2.2.20. Basic Tables

3.2.2.21. Events

3.2.3. Ruby

Ruby elements denote annotations used in some Asian languages. [RUBY]

The Ruby module has been moved into the optional element definitions module. Note that it is normatively required in XHTML 1.1

3.2.4. XHTML Hub Document   (Non-normative)

This is an example base schema document that includes all the other modules to create the complete schema.

3.2.4.1. XHTML 1.1

The hub document included here intends to approximate XHTML 1.1 subject to the requirements given in Section 1.4. This schema should be fully equivalent to the DTD version except for schema-specific additions and changes. This hub document is non-normative and provided only as an example.

3.3. Validity and Conformance

The purpose of any language definition, regardless of its basis on DTDs, XML Schema, or some other representation, is the same: to determine if a specific document instance conforms to the language definition. In XML Schema terms, this means that documents can be validated using the schema. The validation process attempts to determine the document's structural integrity, and the behavior of any XML processor in cases of validation errors is well-defined in the XML 1.0 specification. Therefore the real test of any modularization system for XHTML is whether the resulting schema can be used to determine if any particular XHTML document instance is valid.

This document does not attempt to define conformance beyond the ability to validate the structural integrity of documents. In particular it does not attempt to describe any level of user-agent conformance, as this is not a modularization issue, but an issue for the specification of the language semantics. Conformance to the XML Schema-based modularization framework is strictly defined in terms of document validation. Further levels of conformance are described in the published language specifications themselves.

3.3.1. XHTML Conformance

Schemas defining language variants within the XHTML namespace may be considered to be conformant if they:

• Conform to the existing published language definition for that language variant
• Include all of the modules that are required in this document
• Retain the semantic components of the required elements intact including but not limited to elements, attributes, and attribute values
• Properly identify external elements and attributes with the appropriate XML namespace
• Any combination of the XHTML Schema modules provided that complies with these rules can be considered to be a conformant language variant in the XHTML namespace.

3.3.2. Schema Modularization Conformance

An XML Schema or set of Schema modules can be considered to be conformant to this schema modularization framework if they follow the schema modularization framework conventions described in Section 2.2.

3.3.3. The XHTML Family of Documents

The XHTML Family of Documents is defined as the set of language variants that use the XHTML namespace as the namespace of the root element, which must be <html>.

In order to be a conformant member of the XHTML Family of Documents, an XML Schema or set of schema modules must:

• Define <html> as its root element, and use the XHTML namespace as the root element's namespace.
• Conform to the XHTML Conformance and Schema Modularization Conformance sections of this document.

This class of document definitions includes both XHTML language variants and compound document types using external modules.

3.3.4. Versioning

Versioning of modules that claim conformance to this specification is subject to the framework conventions in Section 2.2. Versioning information should be available in the version block section of each conformant module.

A. References

This appendix is normative.

A.1. Normative References

[APPROACH]

An Approach to the Modularization of XHTL using XML Schema, Rick Jelliffe, Academia Sinica Computing Center  12 December 2000.
See: http://www.ascc.net/~ricko/xhtml.htm

[CSS2]

"Cascading Style Sheets, level 2 (CSS2) Specification", B. Bos, H. W. Lie, C. Lilley, I. Jacobs, 12 May 1998.
Available at: http://www.w3.org/TR/1998/REC-CSS2-19980512

[DOM]

"Document Object Model (DOM) Level 1 Specification", Lauren Wood et al., 1 October 1998.
Available at: http://www.w3.org/TR/1998/REC-DOM-Level-1-19981001

[CATALOG]

Entity Management: OASIS Technical Resolution 9401:1997 (Amendment 2 to TR 9401) Paul Grosso, Chair, Entity Management Subcommittee, SGML Open, 10 September 1997.
See: http://www.oasis-open.org/html/a401.htm

[HTML4]

HTML 4.01 Specification: W3C Recommendation, Dave Raggett, Arnaud Le Hors, Ian Jacobs, 24 December 1999.
See: http://www.w3.org/TR/1999/REC-html401-19991224

[ISO10646]

"Information Technology -- Universal Multiple-Octet Coded Character Set (UCS) -- Part 1: Architecture and Basic Multilingual Plane", ISO/IEC 10646-1:2000. This reference refers to a set of codepoints that may evolve as new characters are assigned to them. Also, this reference assumes that the character sets defined by ISO 10646 and [Unicode] remain character-by-character equivalent. This reference also includes future publications of other parts of 10646 (i.e., other than Part 1) that define characters in planes 1-16.

[MathML]

Mathematical Markup Language 2, David Carlisle, et al. 21 February 200.
See: http://www.w3.org/TR/2001/REC-MathML2-20010221

[RFC1808]

Relative Uniform Resource Locators, R. Fielding.
See: http://www.ietf.org/rfc/rfc1808.txt

[RFC2045]

Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies, N. Freed and N. Borenstein, November 1996. Note that this RFC obsoletes RFC1521, RFC1522, and RFC1590.

[RFC2119]

Key words for use in RFCs to indicate requirement levels, S. Bradner, March 1997.

[RFC3066]

Tags for the Identification of Languages, H. Alvestrand, January 2001.

[RUBY]

Ruby Annotation, Marcin Sawicki, et. al., 16 February 2001
See: http://www.w3.org/TR/ruby

[SGML]

Information Processing -- Text and Office Systems -- Standard Generalized Markup Language (SGML), ISO 8879:1986.
Please consult http://www.iso.ch/cate/d16387.html for information about the standard, or http://www.oasis-open.org/cover/general.html#overview about SGML.

[SMIL]

Synchronized Multimedia Integration Language (SMIL) 1.0 Specification, Philipp Hoschka, 15 June 1998.
See: http://www.w3.org/TR/1998/REC-smil-19980615

[SRGB]

A Standard Default Color Space for the Internet, version 1.10, M. Stokes, M. Anderson, S. Chandrasekar, and R. Motta, 5 November 1996. This document is http://www.w3.org/Graphics/Color/sRGB

[UNICODE]

The Unicode Standard, Version 3.0, The Unicode Consortium, Reading Mass. Addison-Wesley Developers Press, 2000. ISBN 0-201-61633-5. See http://www.unicode.org.

[URI]

Uniform Resource Identifiers (URI): Generic Syntax, T. Berners-Lee, R. Fielding, L. Masinter, August 1998.
See: http://www.ietf.org/rfc/rfc2396.txt. This RFC updates RFC 1738 [URL] and [RFC1808].

[URL]

IETF RFC 1738, Uniform Resource Locators (URL), T. Berners-Lee, L. Masinter, M. McCahill.
See: http://www.ietf.org/rfc/rfc1738.txt

[XHTML1]

XHTML 1.0: The Extensible HyperText Markup Language, Steven Pemberton, et al., 26 January 2000.
See: http://www.w3.org/TR/2000/REC-xhtml1-20000126

[XHTML11]

XHTML 1.1 - Module-based XHTML, Murray Altheim, et. al., 31 May 2001.
See: http://www.w3.org/TR/xhtml11/

[XHTMLBASIC]

XHTML Basic, Mark Baker, et. al., 19 December 2000.
See: http://www.w3.org/TR/xhtml-basic/

[XHTMLMIME]

The application/xhtml+xml Media Type, Mark Baker, IETF, January 2001.
See: http://www.ietf.org/

[XHTMLMOD]

Modularization of XHTML, Murray Altheim, et. al., 10 April 2001
See: http://www.w3.org/TR/xhtml-modularization/

[XML]

Extensible Markup Language (XML) 1.0: Second Edition, Tim Bray, Jean Paoli, C. M. Sperberg-McQueen, 6 October 2000.
See: http://www.w3.org/TR/2000/REC-xml-20001006

[XMLNAMES]

Namespaces in XML, T. Bray, D. Hollander, A. Layman, 14 January 1999.
XML namespaces provide a simple method for qualifying names used in XML documents by associating them with namespaces identified by URI.
Available at: http://www.w3.org/TR/1999/REC-xml-names-19990114

[XMLSCHEMA]

XML Schema Part 1: Structures, Henry S. Thompson, et al., 16 March 2001
See: http://www.w3.org/TR/xmlschema-1

[XMLSCHEMA_COMPOSITION]

XML Schema Part 1: Structures, Henry S. Thompson, et al., 16 March 2001
See: http://www.w3.org/TR/xmlschema-1/#layer2

B. Changes

This appendix is informative.

B.1. Changes to Abstract Modules

• Moved Ruby to Optional

B.2. Changes from DTD Module Implementations

• Reorganized legacy with 2 new modules
• Changed Ruby content model
• Moved nameident into optional
• Modified meaning of ".type" objects
• Added file naming, file structure, module structure, and documentation conventions.
• Changed some attribute value definitions to more restricted types
• Added new "req" module

C. Acknowledgements

This appendix is informative.

The following people provided support and assistance:

• Peter Stark
• Steven Pemberton
• Rick Jelliffe
• Josef Dietl
• Sebastian Schnitzenbaumer
• Malte Wedel
• Jack Herer

D. XHTML Schema Module Implementations

This appendix is normative.

This appendix contains implementations of the modules defined in XHTML Abstract Modules via XML Schema [XMLSCHEMA] when the XML Schema becomes a W3C approved recommendation.

D.1. XHTML Abstract Modules and XML Schema

The DTD modularization framework specification speaks at length on the subject of abstract modules. In brief, an "abstract" module is simply a set of objects, in this case objects within an ordered hierarchy of content objects, which encapsulates all of the features of the objects and assembles them into a coherent set. This set of objects and their properties is independent of its machine representation, and so is the same whether written in DTD module form, as a Schema module, or as a Java class.

The abstract modules described in XHTML-MOD are composed in a functional manner, and each "abstract module" contains data structures that are generally functionally similar. (There is no requirement that modules be created along functional lines; any other method that suits the author's purpose may be used instead.)

The framework described here makes use of the same abstract modules as in XHTML-MOD with few exceptions. In the case of the schema module representation, the relationship between the "abstract" modules and the schema modules is quite close. In each case there is a one-to-one relationship between the abstract and concrete modules (with one exception for the changes to the legacy module) and they share essentially the same names and data structures.

D.2. XHTML Schema Modules

D.2.1. XHTML Hub Document

This is the base schema document that includes all the other modules to create the complete schema.

The hub document included here intends to approximate XHTML 1.1 subject to the requirements given in Requirements. This schema should be fully equivalent to the DTD version except for schema-specific additions and changes. Any document instance considered valid using the XHTML 1.1 DTD must also be valid according to this schema.

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema targetNamespace="http://www.w3.org/1999/xhtml" xmlns="http://www.w3.org/1999/xhtml" xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="unqualified" version="1.1" blockDefault="#all" finalDefault="#all" attributeFormDefault="unqualified">
    <!--

-->
    <xs:annotation>
        <xs:documentation>

             This is the XML Schema driver for XHTML 1.1.
             Please use this formal public identifier to identify it:
            
             "-//W3C//SCHEMA XHTML 1.1//EN"
            
             and this namespace for XHTML elements:
            
             "http://www.w3.org/1999/xhtml"

            @author: Daniel Austin austin.d@ic.grainger.com
            $Id: xhtml-m12n-schema.html,v 1.2 2001/12/19 16:40:03 dom Exp $
        </xs:documentation>
    </xs:annotation>
    <!--

-->
    <xs:annotation>
        <xs:documentation>
           This is XHTML, a reformulation of HTML as a modular XML application
           The Extensible HyperText Markup Language (XHTML)
           Copyright 1998-2001 World Wide Web Consortium
           (Massachusetts Institute of Technology, Institut National de
           Recherche en Informatique et en Automatique, Keio University).
           All Rights Reserved.
        
           Permission to use, copy, modify and distribute the XHTML Schema
           modules and their accompanying xs:documentation for any purpose
           and without fee is hereby granted in perpetuity, provided that the above
           copyright notice and this paragraph appear in all copies.  
          The copyright holders make no representation about the suitability of
           these XML Schema modules for any purpose.
        
           They are provided "as is" without expressed or implied warranty.
        </xs:documentation>
    </xs:annotation>
    <!--

-->
    <xs:annotation>
        <xs:documentation>
             This schema includes two modules, one
            for the XHTML required elements and attributes, 
            and one for the optional elements and attributes.
            In order to modify this schema, you can modify the files
            containing the optional modules. It is not necessary to 
            modify this file. All of the required modules must be 
            included without change for conformance.
        </xs:documentation>
    </xs:annotation>
    <!--

-->
    <xs:include schemaLocation="xhtml-required-1.xsd">
        <xs:annotation>
            <xs:documentation>
                 These modules are required to be included unchanged for 
                XHTML conformance.
                
                 Framework modules:
                        +  notations
                        +  datatypes
                        +  xlink
                        +  events
                        +  common attributes
                        +  common content models
                        +  character entities
                
                 Core required elements modules:
                        +  text
                        +  hypertext
                        +  lists
                        +  ruby
                        +  structural
                
            </xs:documentation>
        </xs:annotation>
    </xs:include>
    <!--

-->
    <xs:include schemaLocation="xhtml-optional-1.xsd">
        <xs:annotation>
            <xs:documentation>
                 Optional Element modules:
                
                        +  Edit
                        +  Bdo
                        +  Presentational
                        +  Link
                        +  Meta
                        +  Base
                        +  Scripting
                        +  Style
                        +  Image
                        +  Client-side image maps
                        +  Server-side image maps
                        +  Param
                        +  Applet
                        +  Object
                        +  Tables
                        +  Forms
                        +  Nameident
                        +  Legacy (includes frames)
                        +  Basic forms
                        +  Basic tables
                
            </xs:documentation>
        </xs:annotation>
    </xs:include>
    <!--

-->
</xs:schema>

D.3. XHTML SCHEMA Modular Framework

In order to take advantage of the XHTML Schema Modules, Schema authors need to define the content model for their language. XHTML provides a variety of tools to ease this effort. They are defined in a set of support modules, instantiated by a main Framework module:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema   
targetNamespace="http://www.w3.org/1999/xhtml"
xmlns="http://www.w3.org/1999/xhtml" 
xmlns:xs="http://www.w3.org/2001/XMLSchema"  
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xsi:schemaLocation="http://www.w3.org/2001/XMLSchema http://www.w3.org/2001/XMLSchema.xsd"
elementFormDefault="unqualified" 
version="1.1"
blockDefault="#all" 
finalDefault="#all" 
attributeFormDefault="unqualified">
<!--

-->
  <xs:annotation>
    <xs:documentation>
        This is the XML Schema Framework module for XHTML
        Please use this formal public identifier to identify it:
        
         "-//W3C//XHTML Modular Framework 1.0//EN"
    
        @author: Daniel Austin austin.d@ic.grainger.com
        $Id: xhtml-m12n-schema.html,v 1.2 2001/12/19 16:40:03 dom Exp $
    </xs:documentation>
      <xs:documentation source="xhtml-copyright-1.xsd"/>
  </xs:annotation>
<!--

-->
  <xs:annotation>
    <xs:documentation> 
         Modular Framework
        
        This required module instantiates the modules needed
        to support the XHTML modularization model, including:
        
        +  notations
        +  datatypes
        +  common attributes
        +  common content models
        +  character entities
    </xs:documentation>
  </xs:annotation>
<!-- 

-->
  <xs:include schemaLocation="framework/xhtml-notations-1.xsd">
    <xs:annotation>
            <xs:documentation>
            Notations module
            Contains XHTML notations for data types 
    </xs:documentation>
  </xs:annotation>
  </xs:include>
<!--

-->
<xs:include schemaLocation="framework/xhtml-datatypes-1.xsd">
  <xs:annotation>
    <xs:documentation>
        Datatypes module
    </xs:documentation>
  </xs:annotation>
  </xs:include>
<!--

-->
<xs:include schemaLocation="framework/xhtml-attribs-1.xsd">
  <xs:annotation>
        <xs:documentation>
        Common attributes module
        </xs:documentation>
    </xs:annotation>
  </xs:include>
<!--

-->
  <xs:include schemaLocation="framework/xhtml11-model-1.xsd">
    <xs:annotation>
            <xs:documentation>
            Common content models module
        </xs:documentation>
    </xs:annotation>
  </xs:include>
<!--

-->
  <!-- Entities are broken in XML Schema -->
  <xs:include schemaLocation="framework/xhtml-charent-1.xsd">
    <xs:annotation>
            <xs:documentation>
            Character entities module

            Notation declarations for Latin 1, 
            Special, and Symbol character entity sets
        </xs:documentation>
    </xs:annotation>
  </xs:include>
<!--

-->
</xs:schema>

Note that the module above references a content model module. This module is defined on a per-document type basis in addition to the document type driver file. The Modular framework also relies upon the following component modules:

D.3.1. XHTML Notations

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema 
targetNamespace="http://www.w3.org/1999/xhtml"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns="http://www.w3.org/1999/xhtml" 
elementFormDefault="unqualified"
attributeFormDefault="unqualified"
blockDefault="#all" 
finalDefault="#all"
version="1.1"
xsi:schemaLocation="http://www.w3.org/2001/XMLSchema http://www.w3.org/2001/XMLSchema.xsd">
<!--

-->
  <xs:annotation>
    <xs:documentation>
        This is the XML Schema module for data type notations for XHTML
        Please use this formal public identifier to identify it:
        "-//W3C//NOTATIONS XHTML Notations 1.0//EN"
        
        
        @author: Daniel Austin austin.d@ic.grainger.com
        $Id: xhtml-m12n-schema.html,v 1.2 2001/12/19 16:40:03 dom Exp $
    </xs:documentation>
    <xs:documentation source="xhtml-copyright-1.xsd"/>
    </xs:annotation>
<!--

-->
<xs:annotation>
    <xs:documentation>
    Notations module
    
    Defines the following notations, many of these imported from 
    other specifications and standards. When an existing FPI is
    known, it is incorporated here.
  </xs:documentation>
  </xs:annotation>
<!--

-->
  <!-- W3C XML 1.0 Recommendation -->
  <xs:notation name="w3c-xml" public="ISO 8879//NOTATION Extensible Markup Language (XML) 1.0//EN"/>
  <!-- XML 1.0 CDATA -->
  <xs:notation name="cdata" public="-//W3C//NOTATION XML 1.0: CDATA//EN"/>
  <!-- SGML Formal Public Identifiers -->
  <xs:notation name="fpi" public="ISO 8879:1986//NOTATION Formal Public Identifier//EN"/>
  <!-- XHTML Notations ... -->
  <!-- Length defined for cellpadding/cellspacing -->
  <!-- nn for pixels or nn% for percentage length -->
  <!-- a single character from [ISO10646] -->
  <xs:notation name="character" public="-//W3C//NOTATION XHTML Datatype: Character//EN"/>
  <!-- a character encoding, as per [RFC2045] -->
  <xs:notation name="charset" public="-//W3C//NOTATION XHTML Datatype: Charset//EN"/>
  <!-- a space separated list of character encodings, as per [RFC2045] -->
  <xs:notation name="charsets" public="-//W3C//NOTATION XHTML Datatype: Charsets//EN"/>
  <!-- media type, as per [RFC2045] -->
  <xs:notation name="contentType" public="-//W3C//NOTATION XHTML Datatype: ContentType//EN"/>
  <!-- comma-separated list of media types, as per [RFC2045] -->
  <xs:notation name="contentTypes" public="-//W3C//NOTATION XHTML Datatype: ContentTypes//EN"/>
  <!-- date and time information. ISO date format -->
  <xs:notation name="datetime" public="-//W3C//NOTATION XHTML Datatype: Datetime//EN"/>
  <!-- a language code, as per [RFC3066] -->
  <xs:notation name="languageCode" public="-//W3C//NOTATION XHTML Datatype: LanguageCode//EN"/>
  <!-- nn for pixels or nn% for percentage length -->
  <xs:notation name="length" public="-//W3C//NOTATION XHTML Datatype: Length//EN"/>
  <!-- space-separated list of link types -->
  <xs:notation name="linkTypes" public="-//W3C//NOTATION XHTML Datatype: LinkTypes//EN"/>
  <!-- single or comma-separated list of media descriptors -->
  <xs:notation name="mediaDesc" public="-//W3C//NOTATION XHTML Datatype: MediaDesc//EN"/>
  <!-- pixel, percentage, or relative -->
  <xs:notation name="multiLength" public="-//W3C//NOTATION XHTML Datatype: MultiLength//EN"/>
  <!-- one or more digits (NUMBER) -->
  <xs:notation name="number" public="-//W3C//NOTATION XHTML Datatype: Number//EN"/>
  <!-- one or more digits (NUMBER) -->
  <xs:notation name="pixels" public="-//W3C//NOTATION XHTML Datatype: Pixels//EN"/>
  <!-- script expression -->
  <xs:notation name="script" public="-//W3C//NOTATION XHTML Datatype: Script//EN"/>
  <!-- textual content -->
  <xs:notation name="text" public="-//W3C//NOTATION XHTML Datatype: Text//EN"/>
  <!-- a Uniform Resource Identifier, see [URI] -->
  <xs:notation name="uri" public="-//W3C//NOTATION XHTML Datatype: URI//EN"/>
  <!-- a space-separated list of Uniform Resource Identifiers, see [URI] -->
  <xs:notation name="uris" public="-//W3C//NOTATION XHTML Datatype: URIs//EN"/>
</xs:schema>

D.3.2. XHTML Datatypes

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema targetNamespace="http://www.w3.org/1999/xhtml" xmlns="http://www.w3.org/1999/xhtml" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.w3.org/2001/XMLSchema http://www.w3.org/2001/XMLSchema.xsd" elementFormDefault="unqualified" version="1.1" blockDefault="#all" finalDefault="#all" attributeFormDefault="unqualified">
  <!--

-->
  <xs:annotation>
    <xs:documentation>
    This is the XML Schema datatypes module for XHTML
    Please use this formal public identifier to identify it:
              "-//W3C//XHTML Datatypes 1.0//EN"
    
    @author: Daniel Austin austin.d@ic.grainger.com
    $Id: xhtml-m12n-schema.html,v 1.2 2001/12/19 16:40:03 dom Exp $
  </xs:documentation>
    <xs:documentation source="xhtml-copyright-1.xsd"/>
  </xs:annotation>
  <!--

-->
  <xs:annotation>
    <xs:documentation>
     Datatypes
     defines containers for the following datatypes, many of
     these imported from other specifications and standards.
  </xs:documentation>
  </xs:annotation>
  <!--

-->
  <!-- nn for pixels or nn% for percentage length -->
  <xs:simpleType name="Length">
    <xs:restriction base="xs:string"/>
  </xs:simpleType>
  <!--

-->
  <!-- space-separated list of link types -->
  <xs:simpleType name="LinkTypes">
    <xs:list itemType="xs:NMTOKEN"/>
  </xs:simpleType>
  <!--

-->
  <!-- single or comma-separated list of media descriptors -->
  <xs:simpleType name="MediaDesc">
    <xs:restriction base="xs:string"/>
  </xs:simpleType>
  <!--

-->
  <!-- pixel, percentage, or relative -->
  <xs:simpleType name="MultiLength">
    <xs:restriction base="xs:string"/>
  </xs:simpleType>
  <!--

-->
  <!-- one or more digits (NUMBER) -->
  <xs:simpleType name="Number">
    <xs:restriction base="xs:nonNegativeInteger"/>
  </xs:simpleType>
  <!--

-->
  <!-- integer representing length in pixels -->
  <xs:simpleType name="Pixels">
    <xs:restriction base="xs:nonNegativeInteger"/>
  </xs:simpleType>
  <!--

-->
  <!-- script expression -->
  <xs:simpleType name="Script">
    <xs:restriction base="xs:string"/>
  </xs:simpleType>
  <!--

-->
  <!-- !!! RGB color expression - this needs a better definition -->
  <xs:simpleType name="Color">
    <xs:restriction base="xs:string"/>
  </xs:simpleType>
  <!--

-->
  <!-- textual content -->
  <xs:simpleType name="Text">
    <xs:restriction base="xs:string"/>
  </xs:simpleType>
  <!--

-->
  <!-- Imported Datatypes  -->
  <!-- a single character from [ISO10646] -->
  <xs:simpleType name="Character">
    <xs:restriction base="xs:string"/>
  </xs:simpleType>
  <!--

-->
  <!-- a character encoding, as per [RFC2045] -->
  <xs:simpleType name="Charset">
    <xs:restriction base="xs:string"/>
  </xs:simpleType>
  <!--

-->
  <!-- a space separated list of character encodings, as per [RFC2045] -->
  <xs:simpleType name="Charsets">
    <xs:list itemType="xs:string"/>
  </xs:simpleType>
  <!--

-->
  <!-- media type, as per [RFC2045] -->
  <xs:simpleType name="ContentType">
    <xs:list itemType="xs:string"/>
  </xs:simpleType>
  <!--

-->
  <!-- comma-separated list of media types, as per [RFC2045] -->
  <xs:simpleType name="ContentTypes">
    <xs:list itemType="xs:string"/>
  </xs:simpleType>
  <!--

-->
  <!-- date and time information. ISO date format -->
  <xs:simpleType name="Datetime">
    <xs:restriction base="xs:normalizedString"/>
  </xs:simpleType>
  <!--

-->
  <!-- formal public identifier, as per [ISO8879] -->
  <xs:simpleType name="FPI">
    <xs:restriction base="xs:normalizedString"/>
  </xs:simpleType>
  <!--

-->
  <!-- a language code, as per [RFC3066] -->
  <xs:simpleType name="LanguageCode">
    <xs:restriction base="xs:NMTOKEN"/>
  </xs:simpleType>
  <!--

-->
  <!-- a Uniform Resource Identifier, see [URI] -->
  <xs:simpleType name="URI">
    <xs:restriction base="xs:anyURI"/>
  </xs:simpleType>
  <!--

-->
  <!-- a space-separated list of Uniform Resource Identifiers, see [URI] -->
  <xs:simpleType name="URIs">
    <xs:list itemType="xs:string"/>
  </xs:simpleType>
  <!--

-->
  <!-- comma-separated list of MultiLength -->
  <xs:simpleType name="MultiLengths">
    <xs:restriction base="xs:string"/>
  </xs:simpleType>
</xs:schema>

D.3.3. XHTML Common Attribute Definitions

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema targetNamespace="http://www.w3.org/1999/xhtml" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns="http://www.w3.org/1999/xhtml" elementFormDefault="unqualified" attributeFormDefault="unqualified" blockDefault="#all" finalDefault="#all" version="1.1" xmlns:xml="http://www.w3.org/XML/1998/namespace">
    <xs:annotation>
        <xs:documentation>

         This is the XML Schema common attributes module for XHTML
          Please use this formal public identifier to identify it:
                  "-//W3C//XHTML Common Attributes 1.0//EN"

            @author: Daniel Austin austin.d@ic.grainger.com
            $Id: xhtml-m12n-schema.html,v 1.2 2001/12/19 16:40:03 dom Exp $
        </xs:documentation>
        <xs:documentation source="xhtml-copyright-1.xsd"/>
    </xs:annotation>
    <!--
    This import brings in the XML language attributes xml:lang and xml:space. 
-->
    <xs:import namespace="http://www.w3.org/XML/1998/namespace" schemaLocation="xml-attribs-1.xsd"/>
    
    <xs:annotation>
        <xs:documentation>

  Common Attributes
    This module declares many of the common attributes 
     for the XHTML Schema
    </xs:documentation>
    </xs:annotation>
    <!--

-->
    <xs:attributeGroup name="id">
        <xs:attribute name="id" type="xs:ID"/>
    </xs:attributeGroup>
    <!--

-->
    <xs:attributeGroup name="class">
        <xs:attribute name="class" type="xs:NMTOKENS"/>
    </xs:attributeGroup>
    <!--

-->
    <xs:attributeGroup name="title">
        <xs:attribute name="title" type="xs:string"/>
    </xs:attributeGroup>
    <!--

-->
    <xs:attributeGroup name="Core.extra.attrib">
        <!-- add your attribute here -->
        <!-- xs:anyAttribute namespace="##other"/ -->
    </xs:attributeGroup>
    <!--

-->
    <xs:attributeGroup name="Core.attrib">
        <xs:attributeGroup ref="id"/>
        <xs:attributeGroup ref="class"/>
        <xs:attributeGroup ref="title"/>
        <xs:attributeGroup ref="Core.extra.attrib"/>
    </xs:attributeGroup>
    <!--

-->
    <xs:attributeGroup name="BIDI"/>
    <!--

-->
    <xs:attribute name="dir">
        <xs:simpleType>
            <xs:restriction base="xs:NMTOKEN">
                <xs:enumeration value="ltr"/>
                <xs:enumeration value="rtl"/>
            </xs:restriction>
        </xs:simpleType>
    </xs:attribute>
    <!--

-->

    <xs:attributeGroup name="I18n.attrib">
        <xs:attributeGroup ref="BIDI"/>
        <xs:attribute ref="xml:lang"/>
    </xs:attributeGroup>
    <!--

-->
    <!-- intrinsic event attributes declared previously -->
    <xs:attributeGroup name="Common.extra">
        <!-- add your attributes here -->
    </xs:attributeGroup>
    <!--

-->
    <xs:attributeGroup name="Common.attrib">
        <xs:attributeGroup ref="Core.attrib"/>
        <xs:attributeGroup ref="I18n.attrib"/>
        <!-- !!! xs:attributeGroup ref="Events.attrib"/ -->
        <xs:attributeGroup ref="Common.extra"/>
    </xs:attributeGroup>
    <!--

-->
</xs:schema>

D.3.4. XHTML Character Entities

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema 
targetNamespace="http://www.w3.org/1999/xhtml"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns="http://www.w3.org/1999/xhtml" 
elementFormDefault="unqualified"
attributeFormDefault="unqualified"
blockDefault="#all" 
finalDefault="#all"
version="1.1"
xsi:schemaLocation="http://www.w3.org/2001/XMLSchema http://www.w3.org/2001/XMLSchema.xsd">
<!--

-->
<xs:annotation>
     <xs:documentation>
        This is the XML Schema Character Entities module for XHTML
        Please use this formal public identifier to identify it:
                   "-//W3C//ENTITIES XHTML Character Entities 1.0//EN"
        
        @author: Daniel Austin austin.d@ic.grainger.com
        $Id: xhtml-m12n-schema.html,v 1.2 2001/12/19 16:40:03 dom Exp $
        </xs:documentation>
        <xs:documentation source="xhtml-copyright-1.xsd"/>
</xs:annotation>
<!--

-->
<xs:annotation>
    <xs:docume