Architecture of the World Wide Web, First Edition

1.1.1. Audience of this Document

This document is intended to inform discussions about issues of Web architecture. The intended audience for this document includes:

1. Participants in W3C Activities; i.e., designers of Web technologies and specifications in W3C
2. Other groups and individuals designing technologies to be integrated into the Web
3. Implementers of W3C specifications
4. Web content authors and publishers

Readers will benefit from familiarity with the Requests for Comments ( RFC ) series from the IETF , some of which define pieces of the architecture discussed in this document.

Note: This document does not distinguish in any formal way the terms "language" and "format." Context determines which term is used. The phrase "specification designer" encompasses language, format, and protocol designers.

1.1.2. Scope of this Document

This document presents the general architecture of the Web. Other groups inside and outside W3C also address specialized aspects of Web architecture, including accessibility, internationalization, device independence, and Web Services. The section on Architectural Specifications includes references.

This document strikes a balance between brevity and precision while including illustrative examples. TAG findings are informational documents that complement the current document by providing more detail about selected topics. This document includes some excerpts from the findings. Since the findings evolve independently, this document also includes references to approved TAG findings. For other TAG issues covered by this document but without an approved finding, references are to entries in the TAG issues list .

Many of the examples in this document involve human activity suppose the familiar Web interaction model where a person follows a link via a user agent, the user agent retrieves and presents data, the user follows another link, etc. This document does not discuss in any detail other interaction models such as voice browsing. For instance, when a graphical user agent running on a laptop computer or hand-held device encounters an error, the user agent can report errors directly to the user through visual and audio cues, and present the user with options for resolving the errors. On the other hand, when someone is browsing the Web through voice input and audio-only output, stopping the dialog to wait for user input may reduce usability since it is so easy to "lose one's place" when browsing with only audio-output. This document does not discuss how the principles, constraints, and good practices identified here apply in all interaction contexts.

1.1.3. Principles, Constraints, and Good Practice Notes

The important points of this document are categorized as follows:

Principle

An architectural principle is a fundamental rule that applies to a large number of situations and variables. Architectural principles include "separation of concerns", "generic interface", "self-descriptive syntax," "visible semantics," "network effect" (Metcalfe's Law), and Amdahl's Law: "The speed of a system is limited by its slowest component."

Constraint

In the design of the Web, some design choices, like the names of the p and li elements in HTML, or the choice of the colon (:) character in URIs, are somewhat arbitrary; if paragraph had been chosen instead of p or asterisk (*) instead of colon, the large-scale result would, most likely, have been the same. Other design choices are more fundamental; these are the focus of this document. Design choices can lead to constraints, i.e., restrictions in behavior or interaction within the system. Constraints may be imposed for technical, policy, or other reasons to achieve certain properties of the system, such as accessibility and global scope, and non-functional properties, such as relative ease of evolution, re-usability of components, efficiency, and dynamic extensibility.

Good practice

Good practice — by software developers, content authors, site managers, users, and specification designers — increases the value of the Web.

This categorization is derived from Roy Fielding's work on "Representational State Transfer" [ REST ].

1.2.1. Independent Specifications

Identification, interaction, and representation are independent (or, "orthogonal", or "loosely coupled") concepts:

• one identifies a resource with a URI. One may publish and use a URI without building any representations of the resource or determining whether any representations are available.
• a generic URI syntax allows agents to function in many cases without knowing specifics of URI schemes.
• in many cases one may change the representation of a resource without disrupting references to the resource.

Independence of specifications facilitates a flexible design that can evolve over time. For example, one may refer to an image with a URI without worrying about the format chosen to represent the image. This independence has allowed the introduction of image formats such as PNG and SVG without disrupting references to image resources.

Independent abstractions benefit from independent specifications. Specifications should clearly indicate those features that simultaneously access information from otherwise independent abstractions. For example a specification should draw attention to a feature that requires information from both the header and the body of a message.

Although the HTTP, HTML, and URI specifications are independent for the most part, they are not completely independent. Experience demonstrates that where they are not, problems have arisen:

• The HTML specification includes a protocol extension of sorts: it specifies how a user agent sends HTML form data to a server (as a URI query string). The design works reasonably well, although there are limitations related to internationalization (see the TAG finding " URIs, Addressability, and the use of HTTP GET and POST " ) and the query string design impinges on the server design. Software developers (for example, of [ CGI ] applications) might have an easier time finding the specification if it were published separately and then cited from the HTTP, URI, and HTML specifications.
• The HTML specification allows content providers to instruct HTTP servers to build response headers from META element instances. This is an abstraction violation; the software developer community would benefit from being able to find all HTTP headers from the HTTP specification (including any associated extension registries and specification updates per IETF process). Perhaps as a result, this feature of the HTML specification is not widely deployed. Furthermore, this design has led to confusion in user agent development. The HTML specification states that META in conjunction with http-equiv is intended for HTTP servers, but many HTML user agents interpret http-equiv='refresh' as a client-side instruction.
• Some content authors use the META / http-equiv approach to declare the character encoding scheme of an HTML document. By design, this is a hint that an HTTP server should emit a corresponding "Content-Type" header field. In practice, the use of the hint in servers is not widely deployed. Furthermore, many user agents use this information to override the "Content-Type" header sent by the server. This works against the principle of authoritative representation metadata .

1.2.2. Extensibility

The information in the Web and the technologies used to represent that information change over time. Some examples of successful technologies designed to allow change while minimizing disruption include:

• the fact that URI schemes are independently specified;
• the use of an open set of Internet media types in mail and HTTP to specify document interpretation;
• the separation of the generic XML grammar and the open set of XML namespaces for element and attribute names;
• extensibility models in Cascading Style Sheets (CSS), XSLT 1.0, and SOAP;
• user agent plug-ins.

Below we discuss the property of "extensibility," exhibited by URIs and some data and message formats, which promotes technology evolution and interoperability.

Language subset : one language is a subset (or, "profile") of a second language if any document in the first language is also a valid document in the second language and has the same interpretation in the second language.

Language extension : one language is an extension of a second language if the second is a language subset of the first (thus, the extension is a superset). Clearly, creating an language extension is better for interoperability than creating an incompatible language.

Ideally, many instances of a superset language can be safely and usefully processed as though they were in the language subset. Languages that exhibit this property are said to be "extensible." Language designers can facilitate extensibility by defining how implementations must handle unknown extensions -- for example, that they be ignored (in some way) or should be considered errors.

For example, from early on in the Web, HTML agents followed the convention of ignoring unknown elements. This choice left room for innovation (i.e., non-standard elements) and encouraged the deployment of HTML. However, interoperability problems arose as well. In this type of environment, there is an inevitable tension between interoperability in the short term and the desire for extensibility. Experience shows that designs that strike the right balance between allowing change and preserving interoperability are more likely to thrive and are less likely to disrupt the Web community. Independent specifications help reduce the risk of disruption.

For further discussion, see the section on versioning and extensibility . See also TAG issue xmlProfiles-29 .

1.2.3. Error Handling

Errors occur in networked information systems. The manner in which they are dealt with depends on application context. A user agent acts on behalf of the user and therefore is expected to help the user understand the nature of errors, and possibly overcome them. User agents that correct errors without the consent of the user are not acting on the user's behalf.

Consent does not necessarily imply that the receiving agent must interrupt the user and require selection of one option or another. The user may indicate through pre-selected configuration options, modes, or selectable user interface toggles, with appropriate reporting to the user when the agent detects an error.

To promote interoperability, specification designers should set expectations about behavior in the face of known error conditions. Experience has led to the following observations about error-handling approaches.

• Protocol designers should provide enough information about the error condition so that an agent can address the error condition. For instance, an HTTP 404 message ("resource not found") is useful because it allows user agents to present relevant information to users, enabling them to contact the representation provider in case of problems.
• Experience with the cost of building a user agent to handle the diverse forms of ill-formed HTML content convinced the designers of the XML specification to require that agents fail upon encountering ill-formed content. Because users are unlikely to tolerate such failures, this design choice has pressured all parties into respecting XML's constraints, to the benefit of all.
• An agent that encounters unrecognized content may handle it in a number of ways, including as an error; see also the section on extensibility and versioning .
• Error behavior that is appropriate for a person may not be appropriate for software. People are capable of exercising judgement in ways that software applications generally cannot. An informal error response may suffice for a person but not for a processor.

See the TAG issues contentTypeOverride-24 and errorHandling-20 .

1.2.4. Protocol-based Interoperability

The Web follows Internet tradition in that its important interfaces are defined in terms of protocols, by specifying the syntax, semantics, and sequence of the messages interchanged. The technology shared among Web agents lasts longer than the agents themselves.

It is common for programmers working with the Web to write code that generates and parses these messages directly. It is less common, but not unusual, for end users to have direct exposure to these messages. It is often desirable to provide users with access to format and protocol details: allowing them to " view source ," whereby they may gain expertise in the workings of the underlying system.

2.1.1. URI Aliases

There are many benefits to ensuring that software can determine, by following specifications, that two URIs refer to the same resource. URI producers should be conservative about the number of different URIs they produce for the same resource, especially when software cannot determine the equivalence of those URIs. For example, the parties responsible for weather.example.com should not use both "http://weather.example.com/Oaxaca" and "http://weather.example.com/oaxaca" to refer to the same resource; software will not detect the equivalence relationship by following specifications.

There may, of course, be good reasons for creating similar-looking URIs. For instance, one might reasonably create URIs that begin with "http://www.example.com/tempo" and "http://www.example.com/tiempo" to provide access to resources by users who speak Italian and Spanish.

Likewise, URI consumers should ensure URI consistency. For instance, when transcribing a URI, agents should not gratuitously escape characters. The term "character" refers to URI characters as defined in section 2 of [ URI ].

When a URI alias does become common currency, the URI owner should use protocol techniques such as server-side redirects to connect the two resources. The community benefits when the URI owner supports both the "unofficial" URI and the alias.

2.2.1. URIs in other Roles

In Web architecture, URIs identify resources. Outside the context of Web architecture specifications, URIs can be useful for other purposes, for example, as database keys. For instance, the organizers of a conference might use "mailto:nadia@example.com" to refer to Nadia. While this usage is not licensed by Web architecture specifications, in the context of the conference, all parties may agree to that local policy and understand one another. Certain properties of URIs, such as their potential for global uniqueness, make them appealing as general-purpose identifiers. In the Web architecture, "mailto:nadia@example.com" identifies an Internet mailbox; that is what is licensed by the "mailto" URI scheme specification. The fact that the URI serves other purposes in non-Web contexts does not lead to URI overloading. URI overloading arises when a URI is used to identify two different resources within the context of Web protocols and formats.

2.4.1. URI Scheme Registration

The Internet Assigned Numbers Authority ( IANA ) maintains a registry [ IANASchemes ] of mappings between URI scheme names and scheme specifications. For instance, the IANA registry indicates that the "http" scheme is defined in [ RFC2616 ]. The process for registering a new URI scheme is defined in [ RFC2717 ].

The use of unregistered URI schemes is discouraged for a number of reasons:

• There is no generally accepted way to locate the scheme specification.
• Someone else may be using the scheme for other purposes.
• One should not expect that general-purpose software will do anything useful with URIs of this scheme beyond URI comparison; the network effect is lost.

Note: Some URI scheme specifications (such as the "ftp" URI scheme specification) use the term "designate" where the current document uses "identify."

TAG issue siteData-36 is about expropriation of naming authority.

2.7.1. Internationalized Identifiers

The integration of internationalized identifiers (i.e., composed of characters beyond those allowed by [ URI ]) into the Web architecture is an important and open issue. See TAG issue IRIEverywhere-27 for discussion about work going on in this area.

2.7.2. Assertion that Two URIs Identify the Same Resource

Emerging Semantic Web technologies, including the "Web Ontology Language (OWL)" [ OWL10 ], define RDF [ RDF10 ] properties such as sameAs to assert that two URIs identify the same resource or functionalProperty to imply it.

One consequence of this direction is that URIs syntactically different can be used to identify the same resource. This means that multiple parties may create representations of the (same) resource, all available for retrieval using multiple URIs. A URI owner's rights (e.g., to provide authoritative representation metadata) extend only to the representations served for requests given that URI.

Note also that to URIs that are sameAs one another does not mean they are interchangeable. For instance, suppose that two different organizations own the URIs "http://weather.example.org/stations/oaxaca#ws17a" and "http://weather.example.com/rdfdump?region=oaxaca&station=ws17a". The URIs might both identify the same resource, a certain collection of weather-measuring equipment shared by the two organizations. Although the URIs might be declared "owl:sameAs" each other, the two URI owners might provide very different content when the URIs are dereferenced.

3.3.1. Media Types and Fragment Identifier Semantics

Per [ URI ], given a URI "U#F", and a representation retrieved by dereferencing URI "U" (which is authoritative), the ( secondary ) resource identified by "U#F" is determined by interpreting "F" according to the specification associated with the Internet media type of the representation data. Thus, in the case of Dirk and Nadia, the authoritative interpretation of the fragment identifier is given by the SVG specification, not the XHTML specification (i.e., the context where the URI appears).

The semantics of a fragment identifier are defined by the set of representations that might result from a retrieval action on the primary resource. The fragment's format and resolution is therefore dependent on the media type [ RFC2046 ] of a potentially retrieved representation, even though such a retrieval is only performed if the URI is dereferenced. If no such representation exists, then the semantics of the fragment are considered unknown and, effectively, unconstrained. Fragment identifier semantics are independent of the URI scheme and thus cannot be redefined by URI scheme specifications.

Interpretation of the fragment identifier during a retrieval action is performed solely by the agent; the fragment identifier is not passed to other systems during the process of retrieval. This means that some intermediaries in the Web architecture (such as proxies) have no interaction with fragment identifiers and that redirection (in HTTP [ RFC2616 ], for example) does not account for them. Note also that since dereferencing a URI (e.g., using HTTP) does not involve sending a fragment identifier to a server or other agent, certain access methods (e.g., HTTP PUT, POST, and DELETE) cannot be used to interact with secondary resources.

As with any URI, use of a fragment identifier component does not imply that a retrieval action will take place. A URI with a fragment identifier may be used to refer to the secondary resource without any implication that the primary resource is accessible or will ever be accessed. One may compare URIs with fragment identifiers without a retrieval action. Parties that draw conclusions about the interpretation of a fragment identifier without retrieving a representation do so at their own risk; such interpretations are not authoritative.

3.3.2. Fragment Identifiers and Multiple Representations

For a given resource, an agent may have the choice between representation data in more than one data format (through HTTP content negotiation, for example). Individual data formats may define their own restrictions on, or structure within, the fragment identifier syntax for specifying different types of subsets, views, or external references that are identifiable as secondary resources by that media type. If the primary resource has multiple representations, as is often the case for resources whose representation is selected based on attributes of the retrieval request ("content negotiation"), then whatever is identified by the fragment should be consistent across all of those representations: each representation should either define the fragment such that it corresponds to the same secondary resource, regardless of how it is represented, or the fragment should be left undefined by the representation (i.e., not found).

Suppose, for example, that the owner of "http://weather.example.com/oaxaca/map#zicatela" provides representations of the resource identified by http://weather.example.com/oaxaca/map using three image formats: SVG, PNG, and JPEG/JFIF. The SVG specification defines semantics for fragment identifiers while the other specifications do not. It is not considered an error that only one of the data formats specifies semantics for the fragment identifier. Because the Web is a distributed system in which formats and agents are deployed in a non-uniform manner, the architecture allows this sort of discrepancy. This design allows content authors to take advantage of new data formats while still ensuring reasonable backward-compatibility for users whose agents do not yet implement them.

URI overloading is one possible consequence of inconsistent fragment identifier semantics.

See related TAG issues httpRange-14 and RDFinXHTML-35 .

3.4.1. Inconsistencies between Metadata and Representation Data

Inconsistencies between the data format of representation data and assigned representation metadata do occur. Examples that have been observed in practice include:

• The actual character encoding of a representation (e.g., "iso-8859-1", specified by the encoding attribute in an XML declaration) is inconsistent with the charset parameter in the representation metadata (e.g., "utf-8", specified by the 'Content-Type' field in an HTTP header).
• The namespace of the root element of XML representation data (e.g., as specified by the "xmlns" attribute) is inconsistent with the value of the 'Content-Type' field in an HTTP header.

Agents should detect such inconsistencies but should not resolve them without the consent of the user; see the section on error handling for more information.

Thus, for example, if the parties responsible for "weather.example.com" mistakenly label the satellite photo of Oaxaca as "image/gif" instead of "image/jpeg", and if Nadia's browser detects a problem, Nadia's browser must not ignore the problem (e.g., by simply rendering the JPEG image) without Nadia's consent. Nadia's browser can notify Nadia of the problem or notify Nadia and take corrective action. Of course, user agent developers should not ignore usability issues when handling this type of error; notification may be discreet, and handling may be tuned to meet the user's preferences. See the TAG finding " Client handling of MIME headers " for more in-depth discussion and examples.

Furthermore, representation providers can help reduce the risk of error through careful assignment of representation metadata (especially that which applies across representations). The section on media types for XML presents an example of reducing the risk of error by providing no metadata about character encoding when serving XML.

3.5.1. Unsafe Interactions and Accountability

It is a breakdown of the Web architecture if agents cannot use URIs to reconstruct a "paper trail" of transaction results, i.e., to refer to receipts and other evidence of accepting an obligation. Indeed, each electronic mail message includes a unique message identifier, one reason why email is so useful for managing accountability (since, for example, email can be copied to public archives). On the other hand, HTTP servers and deployed user agents do not generally keep records of POST transactions, making it difficult for all parties to reconstruct a series of transactions.

There are mechanisms in HTTP, not widely deployed, to remedy this situation. HTTP servers can assign a URI to the results of a POST transaction using the "Content-Location" header (described in section 14.14 of [ RFC2616 ]), and allow authorized parties to retrieve a record of the transaction thereafter via this URI (the value of URI persistence is apparent in this case). User agents can provide an interface for managing transactions where the user agent has incurred an obligation on behalf of the user.

3.6.1. Representation availability

A URI owner may supply zero or more representations of the resource identified by that URI. That agent is also responsible for accepting or rejecting requests to modify the resource identified by that URI, for example, by configuring a server to accept or reject HTTP PUT data based on Internet media type, validity constraints, or other constraints.

For example, the owner of an XML Namespace should provide a <a href="#namespace-documents" shape="rect"> Namespace Document namespace representations ; below we discuss useful characteristics of a Namespace Document. namespace representation.

3.6.2. URI Persistence

There are strong social expectations that once a URI identifies a particular resource, it should continue indefinitely to refer to that resource; this is called URI persistence . URI persistence is a matter of policy and commitment on the part of authorities servicing URIs. The choice of a particular URI scheme provides no guarantee that those URIs will be persistent or that they will not be persistent.

Since representations are used to communicate resource state, persistence is directly affected by how well representations are served. Service breakdowns include:

• Inconsistent representations served. Note the difference between a URI owner changing representations predictably in light of the nature of the resource (the changing weather of Oaxaca) and the URI owner changing representations arbitrarily.
• Improper use of content negotiation, such as serving two images as equivalent through HTTP content negotiation, where one image represents a square and the other a circle.

4.2.1. Versioning

There is typically a (long) transition period during which multiple versions of a format, protocol, or agent are simultaneously in use.

4.2.2. Versioning and XML Namespace Policy

Dirk and Nadia have chosen a particular namespace change policy that allows them to avoid changing the namespace name whenever they make changes that do not affect conformance of deployed content and software. They might have chosen a different policy, for example that any new element or attribute has to belong to a namespace other than the original one. Whatever the chosen policy, it should set clear expectations for users of the format.

As an example of a change policy designed to reflect the variable stability of a namespace, consider the W3C namespace policy for documents on the W3C Recommendation track. The policy sets expectations that the Working Group responsible for the namespace may modify it in any way until a certain point in the process ("Candidate Recommendation") at which point W3C constrains the set of possible changes to the namespace in order to promote stable implementations.

Note that since namespace names are URIs, the owner of a namespace URI has the authority to decide the namespace change policy.

4.2.3. Extensibility

Designers can facilitate the transition process by making careful choices about extensibility during the design of a language or protocol specification.

Application needs determine the most appropriate extension strategy for a specification. For example, applications designed to operate in closed environments may allow specification designers to define a versioning strategy that would be impractical at the scale of the Web. As part of defining an extensibility mechanism, specification designers should set expectations about agent behavior in the face of unrecognized extensions.

Two strategies have emerged as being particularly useful:

1. "Must ignore": The agent ignores any content it does not recognize.
2. "Must understand": The agent treats unrecognized markup as an error condition.

A powerful design approach is for the language to allow either form of extension, but to distinguish explicitly between them in the syntax.

Additional strategies include prompting the user for more input, automatically retrieving data from available links, and falling back to default behavior. More complex strategies are also possible, including mixing strategies. For instance, a language can include mechanisms for overriding standard behavior. Thus, a data format can specify "must ignore" semantics but also allow people to create extensions that override that semantics in light of application needs (for instance, with "must understand" semantics for a particular extension).

Extensibility is not free. Providing hooks for extensibility is one of many requirements to be factored into the costs of language design. Experience suggests that the long term benefits of extensibility generally outweigh the costs.

4.2.4. Composition of Data Formats

Many modern data format include mechanisms for composition. For example:

• It is possible to embed text comments in some image formats, such as JPEG/JFIF. Although these comments are embedded in the containing data, they have little or no effect on the display of the image.
• There are container formats such as SOAP which fully expect to be composed from multiple namespaces but which provide an overall semantic relationship of message envelope and payload.
• RDF allows well-defined mixing of vocabularies, and allows text and XML to be used as a data type values within a statement having clearly defined semantics.

These relationships can be mixed and nested arbitrarily. In principle, a SOAP message can contain an SVG image that contains an RDF comment which refers to a vocabulary of terms for describing the image.

Note however, that for general XML there is no semantic model that defines the interactions within XML documents with elements and/or attributes from a variety of namespaces. Each application must define how namespaces interact and what effect the namespace of an element has on the element's ancestors, siblings, and descendants.

See TAG issues mixedUIXMLNamespace-33 , xmlFunctions-34 , and RDFinXHTML-35 .

4.4.1. URI References

Links are commonly expressed using URI references (defined in section 4.2 of [ URI ]), which may be combined with a base URI to yield a usable URI. Section 5.1 of [ URI ] explains different mechanisms for establishing a base URI for a resource and establishes a precedence among the various mechanisms. For instance, the base URI may be a URI for the resource, or specified in a representation (see the base elements provided by HTML and XML, and the HTTP 'Content-Location' header). See also the section on links in XML .

Agents resolve a URI reference before using the resulting URI to interact with another agent. URI references help in content management by allowing content authors to design a representation locally, i.e., without concern for which global identifier may later be used to refer to the associated resource.

4.5.1. When to Use an XML-Based Format

XML defines textual data formats that are naturally suited to describing data objects which are hierarchical and processed in a chosen sequence. It is widely, but not universally, applicable for data formats; an audio or video format, for example, is unlikely to be well suited to expression in XML. Design constraints that would suggest the use of XML include:

1. Requirement for a hierarchical structure.
2. The data's usefulness should outlive the tools currently used to process it (though obviously XML can be used for short-term needs as well).
3. Ability to support internationalization in a self-describing way that makes confusion over coding options unlikely.
4. Early detection of encoding errors with no requirement to "work around" such errors.
5. A high proportion of human-readable textual content.
6. Potential composition of the data format with other XML-encoded formats.

4.5.2. Links in XML

Sophisticated linking mechanisms have been invented for XML formats. XPointer allows links to address content that does not have an explicit, named anchor. XLink is an appropriate specification for representing links in hypertext XML applications. XLink allows links to have multiple ends and to be expressed either inline or in "link bases" stored external to any or all of the resources identified by the links it contains.

Designers of XML-based formats should consider using XLink and, for defining fragment identifier syntax, using the XPointer framework and XPointer element() Schemes.

See TAG issue xlinkScope-23 .

4.5.3. XML Namespaces

How can one ensure that there are no naming conflicts when elements from different XML-based data formats are mixed? For example, suppose that one designer defines the para element in an XML format to identify paragraphs, and another designer defines the para element in a second format to identify parachutes. "Namespaces in XML" [ XMLNS ] provides a mechanism for establishing globally unique names.

Specification designers who declare namespaces thus provide a global context for instances of the data format. Establishing this global context allows those instances (and portions thereof) to be re-used and combined in novel ways not yet imagined. Failure to provide a namespace makes such re-use more difficult, perhaps impractical in some cases.

Attributes are always scoped by the element on which they appear. An attribute that is "global," that is, one that might meaningfully appear on elements of any type, including elements in other namespaces, should be explicitly placed in a namespace. Local attributes, ones associated with only a particular element type, need not be included in a namespace since their meaning will always be clear from the context provided by that element.

The xsi:type attribute, provided by W3C XML Schema for use in XML instance documents, is an example of a global attribute. It can be used by authors of any vocabulary to make an assertion in instance data about the type of the element on which it appears. The type attribute occurs in the W3C XML Schema namespace "http://www.w3.org/2001/XMLSchema" and must always be fully qualified. The frame attribute on an HTML table is an example of a local attribute. There is no value in placing that attribute in a namespace since the attribute is unlikely to be useful on an element other than an HTML table.

Applications that rely on DTD processing must impose additional constraints on the use of namespaces. DTDs perform validation based on the lexical form of the element and attribute names in the document. This makes prefixes syntactically significant in ways that are not anticipated by [ XMLNS ].

4.5.4. <a name="namespace-documents" id="namespace-documents" shape="rect"> Namespace Documents Representation

There are many reasons to provide information about a namespace. A person might want to:

• understand its purpose,
• learn how to use the markup vocabulary in the namespace,
• find out who controls it,
• request authority to access schemas or collateral material about it, or
• report a bug or situation that could be considered an error in some collateral material.

A processor might want to:

• retrieve a schema, for validation,
• retrieve a style sheet, for presentation, or
• retrieve ontologies, for making inferences.

In general, there is no established best practice for creating a namespace document. representation. Application expectations will influence what data format or formats are used to create a namespace document. representation. Application expectations will also influence whether relevant information appears in the namespace document representation itself or is referenced from it.

For example, the following are examples of formats used to create namespace documents: representations: [ OWL10 ], [ RDDL ], [ XMLSCHEMA ], and [ XHTML11 ]. Each of these formats meets different requirements described above for satisfying the needs of an agent that wants more information about the namespace. Note, however, issues related to fragment identifiers and multiple representations if content negotiation is used with namespace documents. representations.

See TAG issues namespaceDocument-8 and abstractComponentRefs-37 .

4.5.5. QNames in XML

Section 3 of "Namespaces in XML" [ XMLNS ] provides a syntactic construct known as a QName for the compact expression of qualified names in XML documents. A qualified name is a pair consisting of a URI, which names a namespace, and a local name placed within that namespace. "Namespaces in XML" provides for the use of QNames as names for XML elements and attributes.

Other specifications, starting with [ XSLT10 ], have employed the idea of using QNames in contexts other than element and attribute names, for example in attribute values and in element content. However, general XML processors cannot reliably recognize QNames as such when they are used in attribute values and in element content; for example, the syntax of QNames overlaps with that of URIs. Experience has also revealed other limitations to QNames, such as losing namespace bindings after XML canonicalization.

For more information, see the TAG finding " Using QNames as Identifiers in Content " .

Because QNames are compact, some specification designers have adopted the same syntax as a means of identifying resources. Though convenient as a shorthand notation, this usage has a cost. There is no single, accepted way to convert a QName into a URI or vice versa. Although QNames are convenient, they do not replace the URI as the identification mechanism of the Web. The use of QNames to identify Web resources without providing a mapping to URIs is inconsistent with Web architecture.

For examples of QName-to-URI mappings, see [ RDF10 ]. See also TAG issues rdfmsQnameUriMapping-6 , qnameAsId-18 , and abstractComponentRefs-37 .

4.5.6. XML ID Semantics

Consider the following fragment of XML: <section name="foo"> . Does the section element have what the XML Recommendation refers to as the ID foo ? One cannot answer this question by examining the element and its attributes alone. In XML, the quality of "being an ID" is associated with the type of an attribute, not its name. Finding the IDs in a document requires additional processing.

1. Processing the document with a processor that recognizes DTD attribute list declarations (in the external or internal subset) might reveal a declaration that identifies the name attribute as an ID. Note: This processing is not necessarily part of validation. A non-validating, DTD-aware processor can perform ID assignment.
2. Processing the document with a W3C XML schema might reveal an element declaration that identifies the name attribute as an xs:ID .
3. In practice, processing the document with another schema language, such as RELAX NG [ RELAXNG ], might reveal the attributes declared to be of ID in the XML Schema sense. Many modern specifications begin processing XML at the Infoset [ INFOSET ] level and do not specify normatively how an Infoset is constructed. For those specifications, any process that establishes the ID type in the Infoset (and Post Schema Validation Infoset ( PSVI ) defined in [ XMLSCHEMA ]) may usefully identify the attributes of type ID.
4. In practice, applications may have independent means of specifying ID-ness as provided for and specified in the XPointer specification.

To further complicate matters, DTDs establish the ID type in the Infoset whereas W3C XML Schema produces a PSVI but does not modify the original Infoset. This leaves open the possibility that a processor might only look in the Infoset and consequently would fail to recognize schema-assigned IDs.

The TAG expects to continue to work with other groups to help resolve open questions about establishing "ID-ness" in XML formats. See TAG issue xmlIDSemantics-32 .

4.5.7. Media Types for XML

RFC 3023 defines the Internet media types "application/xml" and "text/xml", and describes a convention whereby XML-based data formats use Internet media types with a "+xml" suffix, for example "image/svg+xml".

These Internet media types create two problems: First, for data identified as "text/*", Web intermediaries are allowed to "transcode", i.e., convert one character encoding to another. Transcoding may make the self-description false or may cause the document to be not well-formed.

Second, representations whose Internet media types begin with "text/" are required, unless the charset parameter is specified, to be considered to be encoded in US-ASCII. Since the syntax of XML is designed to make documents self-describing, it is good practice to omit the charset parameter, and since XML is very often not encoded in US-ASCII, the use of "text/" Internet media types effectively precludes this good practice.

4.5.8. Fragment Identifiers in XML

The section on media types and fragment identifier semantics discusses the interpretation of fragment identifiers. Designers of an XML-based data format specification should define the semantics of fragment identifiers in that format. The XPointer Framework [ XPTRFR ] provides an interoperable starting point.

When the media type assigned to representation data is "application/xml", there are no semantics defined for fragment identifiers, and authors should not make use of fragment identifiers in such data. The same is true if the assigned media type has the suffix "+xml" (defined in "XML Media Types" [ RFC3023 ]), and the data format specification does not specify fragment identifier semantics. In short, just knowing that content is XML does not provide information about fragment identifier semantics.

Many people assume that the fragment identifier #abc , when referring to XML data, identifies the element in the document with the ID "abc". However, there is no normative support for this assumption.

See TAG issue fragmentInXML-28 .

6.1. Architectural Specifications

ATAG10

Authoring Tool Accessibility Guidelines 1.0 , J. Treviranus, C. McCathieNevile, I. Jacobs, J. Richards, Editors, W3C Recommendation, 3 February 2000, http://www.w3.org/TR/2000/REC-ATAG10-20000203 . Latest version available at http://www.w3.org/TR/ATAG10 .

CHARMOD

Character Model for the World Wide Web 1.0: Fundamentals , T. Texin, M. J. Dürst, F. Yergeau, R. Ishida, M. Wolf, Editors, W3C Working Draft (work in progress), 25 February 2004, http://www.w3.org/TR/2004/WD-charmod-20040225/ . Latest version available at http://www.w3.org/TR/charmod/ .

DIPRINCIPLES

Device Independence Principles , R. Gimson, Editor, W3C Working Group Note, 1 September 2003, http://www.w3.org/TR/2003/NOTE-di-princ-20030901/ . Latest version available at http://www.w3.org/TR/di-princ/ .

EXTLANG

Web Architecture: Extensible Languages , T. Berners-Lee, D. Connolly, 10 February 1998. This W3C Note is available at http://www.w3.org/TR/1998/NOTE-webarch-extlang-19980210.

Fielding

Principled Design of the Modern Web Architecture , R.T. Fielding and R.N. Taylor, UC Irvine. In Proceedings of the 2000 International Conference on Software Engineering (ICSE 2000), Limerick, Ireland, June 2000, pp. 407-416. This document is available at http://www.ics.uci.edu/~fielding/pubs/webarch_icse2000.pdf.

QA

QA Framework: Specification Guidelines , D. Hazaël-Massieux, L. Henderson, L. Rosenthal, Editors, W3C Candidate Recommendation (work in progress), 10 November 2003, http://www.w3.org/TR/2003/CR-qaframe-spec-20031110/ . Latest version available at http://www.w3.org/TR/qaframe-spec/ .

RFC1958

IETF RFC 1958: Architectural Principles of the Internet , B. Carpenter, June 1996. Available at http://www.ietf.org/rfc/rfc1958.txt.

UAAG10

User Agent Accessibility Guidelines 1.0 , I. Jacobs, J. Gunderson, E. Hansen, Editors, W3C Recommendation, 17 December 2002, http://www.w3.org/TR/2002/REC-UAAG10-20021217/ . Latest version available at http://www.w3.org/TR/UAAG10/ .

WCAG20

Web Content Accessibility Guidelines 2.0 , B. Caldwell, W. Chisholm, G. Vanderheiden, J. White, Editors, W3C Working Draft (work in progress), 11 March 2004, http://www.w3.org/TR/2004/WD-WCAG20-20040311/ . Latest version available at http://www.w3.org/TR/WCAG20/ .

WSA

Web Services Architecture , D. Orchard, D. Booth, H. Haas, F. McCabe, E. Newcomer, M. Champion, C. Ferris, Editors, W3C Working Group Note, 11 February 2004, http://www.w3.org/TR/2004/NOTE-ws-arch-20040211/ . Latest version available at http://www.w3.org/TR/ws-arch/ .

XAG

XML Accessibility Guidelines , D. Dardailler, S. B. Palmer, C. McCathieNevile, Editors, W3C Working Draft (work in progress), 3 October 2002, http://www.w3.org/TR/2002/WD-xag-20021003 . Latest version available at http://www.w3.org/TR/xag .