Representing Content in RDF 1.0

Abstract

This document is a specification for a vocabulary to represent content in the Resource Description Framework (RDF). This vocabulary is intended to provide a flexible framework within different usage scenarios to semantically represent any type of content, be it on the Web or in local storage media. For example, it can be used by web quality assurance tools such as web accessibility evaluation tools to record a representation of the assessed web content, including text, images, or other types of formats. In many cases, it can be used together with HTTP Vocabulary in RDF 1.0, which allows quality assurance tools to record the HTTP headers that have been exchanged between a client and a server. This is particularly useful for quality assurance testing, conformance claims, and reporting languages like the W3C Evaluation And Report Language (EARL).

1 Introduction

This document is the specification for a vocabulary to represent Content in the Resource Description Framework (RDF). There is a wide variety of scenarios (see section below) where a representation of any type of content, either on the Web or in any local storage media, is necessary. This specification provides an RDF application that allows to present semantically such content. The vocabulary is built in a flexible manner, thus there are no limitations known at the time of writing this specification. It also provides opportunities for extensions to match particular needs of its users.

This document assumes the following background knowledge:

• Basic knowledge of the Extensible Markup Language (XML) [XML] and its associated technologies.
• Basic knowledge about the Semantic Web and RDF. For references, consult [RDF], [RDF-PRIMER] and [RDFS].

The terms defined by this document can be used as part of the W3C Evaluation And Report Language (EARL) and in other contexts too. Developer Guide for Evaluation and Report Language (EARL) 1.0 explains how to implement and use EARL, including conformance requirements for software tools.

Although the concepts of the Semantic Web are simple, their abstraction with RDF is known to bring difficulties to beginners. It is recommended to read carefully the aforementioned references and other tutorials found on the Web. It must be also borne in mind that RDF is primarily targeted to be machine processable, and therefore, some of its expressions are not very intuitive for developers used to work with XML only. The examples will be serialized using the abbreviated RDF/XML notation.

The keywords must, required, recommended, should, may, and optional are used in accordance with [RFC2119].

For limitations of this vocabulary, see section 5.

1.1 Namespaces

Table 1 presents the namespaces typically used by this vocabulary. The core namespace has the URI https://www.w3.org/2011/content# and the prefix cnt. The prefix notation presents the typical conventions used in the Web and in this document to denote a given namespace, and can be freely modified.

1.2 Use cases

As stated earlier, this framework is designed in an open way to facilitate different implementation scenarios. The origin of the application comes from vocabularies describing testing scenarios like the Evaluation And Report Language (EARL) [EARL]. Typical applications could be:

• Applications dealing with retrieval, editing and storage of content. For example, an archiving application could store in a database annotated media content that includes a serialization of the media files with this vocabulary.
• Applications dealing with the exchange of text documents and other types of media, like Web Services. For example, an AJAX application could exchange document fragments and images with a Web server to react to different user actions.
• Applications dealing with the testing and/or repair of content. For example, an accessibility testing tool could store together with the results of a compliance test, the tested Web resources to ensure that the correct version of the tested subject is available to the developers.

2 Classes

This section presents a description of the classes of this RDF vocabulary. We present every class together with its subclasses. We also include whenever relevant short snippets and examples.

2.1 Content Class

The cnt:Content class is an overarching class for any content that could be found on the Web, in an Intranet or in local storage media, for example. It is recommended always to use one of its subclasses. There is no restriction within the vocabulary scope on what can be represented with this class: textual content, XML files, binary files (e.g., images or movies), etc.

There are three subclasses from the Content class: cnt:ContentAsBase64, cnt:ContentAsText and cnt:ContentAsXML.

In order to connect resources with different cnt:Content sub-types with each other, use the dct:hasFormat and dct:isFormatOf properties to point to each other. E.g. if there is an XML resource transmitted via HTTP, the original version would be a cnt:ContentAsBase64 resource. But cnt:ContentAsText and cnt:ContentAsXML resource could also be created and point to the cnt:ContentAsBase64 resource.

Examples

<cnt:ContentAsBase64 rdf:ID="xml0">
  <!-- ... -->
  <dct:isFormatOf rdf:resource="https://www.example.org/index.html"/>
  <dct:hasFormat rdf:resource="#xml1"/>
  <dct:hasFormat rdf:resource="#xml2"/>
</cnt:ContentAsBase64>
<cnt:ContentAsText rdf:ID="xml1">
  <!-- ... -->
  <dct:isFormatOf rdf:resource="https://www.example.org/index.html"/>
  <dct:hasFormat rdf:resource="#xml0"/>
  <dct:hasFormat rdf:resource="#xml2"/>
</cnt:ContentAsText>
<cnt:ContentAsXML rdf:ID="xml2">
  <!-- ... -->
  <dct:isFormatOf rdf:resource="https://www.example.org/index.html"/>
  <dct:hasFormat rdf:resource="#xml0"/>
  <dct:hasFormat rdf:resource="#xml1"/>
</cnt:ContentAsXML>

Related Properties

• Domain of:
  • dct:hasFormat
  • dct:isFormatOf
• Range of: none

2.1.1 ContentAsBase64 Class

The cnt:ContentAsBase64 class is a subclass of the cnt:Content class for Base64 encoded binary content (as defined by [RFC2045]) and can be used for any type of content, although its more typical use case is for binary files.

Related Properties

• Domain of:
  • cnt:bytes
  • cnt:characterEncoding
• Range of: none

Examples

<cnt:ContentAsBase64>
  <cnt:bytes>77+9UE5HDQoaCgAAAA1JSERSAAAASAAAADAIAwAAAO+{...}</cnt:bytes>
  <dct:isFormatOf rdf:resource="https://www.w3.org/Icons/w3c_home.png"/>
</cnt:ContentAsBase64>

2.1.2 ContentAsText Class

The cnt:ContentAsText class is a subclass of the cnt:Content class for any type of textual content.

Related Properties

• Domain of:
  • cnt:chars
  • cnt:characterEncoding
• Range of: none

Examples

<cnt:ContentAsText>
  <cnt:characterEncoding>UTF-8</cnt:characterEncoding>
  <cnt:chars>body {
  color: #000;
  background: #fff
}
h1 {
  font-size: 1.6em
}
h2 {
  font-size: 1.3em
}</cnt:chars>
  <dct:isFormatOf rdf:resource="https://example.org/example.css"/>
</cnt:ContentAsText>

2.1.3 ContentAsXML Class

The cnt:ContentAsXML class is a subclass of the cnt:Content class only for wellformed XML content.

Related Properties

• Domain of:
  • cnt:characterEncoding
  • cnt:declaredEncoding
  • cnt:dtDecl
  • cnt:leadingMisc
  • cnt:rest
  • cnt:standalone
  • cnt:version
• Range of: none

See the Mapping between the Document Object Model (DOM) and the Content-in-RDF vocabulary.

Examples

<html xmlns="https://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
  <head>
    <title>The title</title>
  </head>
  <body>
    <p>Some paragraph.</p>
  </body>
</html>

<cnt:ContentAsXML>
  <cnt:rest rdf:parseType="Literal"><html xmlns="https://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
  <head>
    <title>The title</title>
  </head>
  <body>
    <p>Some paragraph.</p>
  </body>
</html></cnt:rest>
  <dct:isFormatOf rdf:resource="https://example.org/example203.html"/>
</cnt:ContentAsXML>

For the use of leadingMisc and dtDecl see Appendix A: A practical example.

2.2 DoctypeDecl Class

A document type declaration. This class is normally used in conjunction with the ContentAsXML class, when the corresponding XML resource contains a document type declaration. The relation is expressed via the dtDecl property.

Related Properties

• Domain of:
  • cnt:doctypeName
  • cnt:internalSubset
  • cnt:publicId
  • cnt:systemId
• Range of:
  • cnt:dtDecl

See the Mapping between the Document Object Model (DOM) and the Content-in-RDF vocabulary.

Examples

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  "https://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

<cnt:DoctypeDecl rdf:ID="dtd0">
  <cnt:doctypeName>html</cnt:doctypeName>
  <cnt:publicId>-//W3C//DTD XHTML 1.0 Strict//EN</cnt:publicId>
  <cnt:systemId>https://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd</cnt:systemId>
</cnt:DoctypeDecl>

3 Properties

This section presents a description of the properties of this RDF vocabulary.

3.1 bytes Property

Character string representing the Base64 encoded byte sequence of the given content.

Domain:

cnt:ContentAsBase64

Range:

Base64 encoded bytes (https://www.w3.org/2001/XMLSchema#base64Binary)

3.2 characterEncoding Property

The character encoding.

• When used with ContentAsBase64: If the byte sequence was created from a given character sequence this property can be used to store the character encoding that was applied to create the byte sequence.
• When used with ContentAsText: If the character sequence was created from a given byte sequence this property can be used to store the character encoding that was applied to create the character sequence.
• When used with ContentAsXML: If the parser's input character stream was created from a given byte stream this property can be used to store the character encoding that was applied to create the character stream. Note: This is the used character encoding, not the one declared in an XML declaration.

Domain:

cnt:Content

Range:

Literal

3.3 chars Property

The character sequence of the given content.

Domain:

cnt:ContentAsText

Range:

Literal

3.4 declaredEncoding Property

The character encoding specified in the XML declaration.

Domain:

cnt:ContentAsXML

Range:

Literal

3.5 doctypeName Property

The document type name.

Domain:

cnt:DoctypeDecl

Range:

Literal

3.6 dtDecl Property

This property relates an XML Content to its Document Type Declaration.

Domain:

cnt:ContentAsXML

Range:

cnt:DoctypeDecl

3.7 internalSubset Property

The internal subset of a document type declaration.

Domain:

cnt:DoctypeDecl

Range:

Literal

3.8 leadingMisc Property

The part of the XML information items (whitespace, comments and processing instructions) following the XML declaration and preceding the document type declaration if there is one.

Domain:

cnt:ContentAsXML

Range:

XML Literal

3.9 publicId Property

The formal public identifier of a document type declaration.

Domain:

cnt:DoctypeDecl

Range:

Literal

3.10 rest Property

• The part of the XML information items following the document type declaration if there is a document type declaration, or
• the part following the XML declaration if there is no document type declaration, or
• the whole XML information items if there is neither XML declaration nor document type declaration.

It contains comments, processing instructions and the root element.

Domain:

cnt:ContentAsXML

Range:

XML Literal

3.11 standalone Property

The standalone document declaration.

Domain:

cnt:ContentAsXML

Range:

Literal

3.12 systemId Property

The system identifier of a document type declaration.

Domain:

cnt:DoctypeDecl

Range:

Literal typed as URI (https://www.w3.org/2001/XMLSchema#anyURI)

3.13 version Property

The XML version specified in the XML declaration.

Domain:

cnt:ContentAsXML

Range:

Literal

5 Usage scenarios

We have identified some situations to make clear when to create which type of content resources. The following are only recommendations and are non-normative:

Situation A: byte sequence of non-text content

This includes images, multimedia, or other non-text resources. The byte sequence is recorded in Base64 format and represented as a literal using the cnt:bytes property of the cnt:ContentAsBase64. Non-text content should not be represented using cnt:ContentAsText.

Situation B: byte sequence of text content with appropriate character encoding information

This includes HTML, CSS, client-side script, or other text-based resources. Given the byte sequence of text content (byteSeq) received from a Web server and an appropriate character encoding (ce). byteSeq is recorded in Base64 format and represented as a literal using the cnt:bytes property of the cnt:ContentAsBase64.

After transforming the byteSeq to a character sequence charSeq using character encoding ce, charSeq is represented as a literal using the cnt:chars property of the cnt:ContentAsText and ce as a literal usind the cnt:characterEncoding property.

Situation C: byte sequence of text content with inappropriate character encoding information

Given the byte sequence of text content (byteSeq) received from a Web server and an inappropriate character encoding (ce). byteSeq is recorded in Base64 format and represented as a literal using the cnt:bytes property of the cnt:ContentAsBase64. Because transforming byteSeq to a character sequence charSeq using character encoding ce fails, no cnt:ContentAsText resource can be created.

Situation D: character sequence of text content with appropriate character encoding information

Given the character sequence of text content (charSeq) created in memory and an appropriate character encoding (ce). A cnt:ContentAsText resource may be created with a cnt:chars property with an object literal created from charSeq. After transforming charSeq to byte sequence byteSeq using character encoding ce, a cnt:ContentAsBase64 resource may be created with cnt:bytes property with an object literal byteSeq and cnt:characterEncoding property with an object literal ce.

Situation E: byte sequence of XML content with appropriate character encoding information

Given the byte sequence of wellformed XML content (byteSeq) received from a Web server and an appropriate character encoding (ce). cnt:ContentAsBase64 and cnt:ContentAsText resources may be created as in situation B. Additionally, an cnt:ContentAsXML resource may be created.

Situation F: Document Object Model (DOM) changed in memory

Given a DOM Document in memory, originally created by parsing some XML source, but afterwards changed by DOM operations. A cnt:XMLDecl resource may be created from the information in the Document node itself (version, declaredEncoding and standalone), and a cnt:DoctypeDecl resource from the information in the DocumentType node. A cnt:ContentAsXML resource may be created after serializing the relevant child nodes of the Document node to create object literals for cnt:leadingMisc (serialize Comment and ProcessingInstruction nodes preceding a DocumentType node) and cnt:rest (serialize nodes following a DocumentType node). See the Mapping between the Document Object Model (DOM) and Content-in-RDF properties.

6 Limitations of the vocabulary

The vocabulary provides a framework that allows the representation of any type of content. Of course, there are many possibilities for extensions that will allow the inclusion of additional metadata, like, e.g., that included in some multimedia formats. Typical scenarios for extensions could be:

• Classes to specify the Document Object Model (DOM) of XML or HTML documents.
• Classes to specify the metadata of multimedia content like audio or image files.
• Properties to ensure the integrity of data by providing some kind of checksum algorithm.

However, at the point of writing this specification, the Working Group has decided to provide the basic framework that will support the immediate needs of vocabularies using this specification like the Evaluation and Report Language (EARL) [EARL], leaving the room open for further extensions as new use cases are presented to us.

Appendix A: A practical example

To understand the versatility of the vocabulary, let us assume we have a given XHTML page containing an XML declaration, a comment preceding a document type declaration and some XHTML elements.

<?xml version="1.0" encoding="UTF-8" standalone="no" ?>
<!-- this is a comment -->
<!DOCTYPE html "-//W3C//DTD XHTML 1.0 Strict//EN"
  "https://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="https://www.w3.org/1999/xhtml" lang="en">
  <head>
    <title>The title</title>
  </head>
  <body>
    <p>Some paragraph.</p>
  </body>
</html>

<rdf:RDF
    xmlns:rdf="https://www.w3.org/1999/02/22-rdf-syntax-ns#"
    xmlns:cnt="https://www.w3.org/2011/content#">
  <cnt:ContentAsText>
    <cnt:chars>&lt;?xml version="1.0" encoding="UTF-8" standalone="no" ?&gt;
&lt;!-- this is a comment --&gt;
&lt;!DOCTYPE html "-//W3C//DTD XHTML 1.0 Strict//EN"
  "https://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"&gt;
&lt;html xmlns="https://www.w3.org/1999/xhtml" xml:lang="en" lang="en"&gt;
  &lt;head&gt;
    &lt;title&gt;The title&lt;/title&gt;
  &lt;/head&gt;
  &lt;body&gt;
    &lt;p&gt;Some paragraph.&lt;/p&gt;
  &lt;/body&gt;
&lt;/html&gt;</cnt:chars>
    <dct:isFormatOf rdf:resource="https://example.org/example207.html"/>
  </cnt:ContentAsText>
</rdf:RDF>

<rdf:RDF
    xmlns:rdf="https://www.w3.org/1999/02/22-rdf-syntax-ns#"
    xmlns:cnt="https://www.w3.org/2011/content#"
    xml:base="https://example.org/example208.html">
  <cnt:DoctypeDecl rdf:ID="dtd0">
    <cnt:systemId>https://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd</cnt:systemId>
    <cnt:publicId>-//W3C//DTD XHTML 1.0 Strict//EN</cnt:publicId>
    <cnt:doctypeName>html</cnt:doctypeName>
  </cnt:DoctypeDecl>
  <cnt:ContentAsXML>
    <cnt:version>1.0</cnt:version>
    <cnt:declaredEncoding>UTF-8</cnt:declaredEncoding>
    <cnt:standalone>no</cnt:standalone>
    <cnt:leadingMisc rdf:parseType="Literal"><!-- this is a comment --></cnt:leadingMisc>
    <cnt:dtDecl rdf:resource="#dtd0" />
    <cnt:rest rdf:parseType="Literal"><html xmlns="https://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
  <head>
    <title>The title</title>
  </head>
  <body>
    <p>Some paragraph.</p>
  </body>
</html></cnt:rest>
    <dct:isFormatOf rdf:resource="https://example.org/example208.html"/>
  </cnt:ContentAsXML>
</rdf:RDF>

Appendix B: Terms

The following terms are defined by this specification:

Classes

Properties

Appendix C: Mapping between the Document Object Model (DOM) and Content-in-RDF properties

Appendix D: References

[EARL]

Evaluation and Report Language (EARL) 1.0 Schema. W3C Working Draft 28 April 2009.
https://www.w3.org/TR/EARL10/

[RDF]

Resource Description Framework (RDF) Model and Syntax Specification. W3C Recommendation, 22 February 1999.
https://www.w3.org/TR/1999/REC-rdf-syntax-19990222/

[RDF-PRIMER]

RDF Primer. W3C Recommendation, 10 February 2004.
https://www.w3.org/TR/rdf-primer/

[RDFS]

RDF Vocabulary Description Language 1.0: RDF Schema. W3C Recommendation 10 February 2004.
https://www.w3.org/TR/rdf-schema/

[RDF-XML]

RDF/XML Syntax Specification (Revised). W3C Recommendation 10 February 2004.
https://www.w3.org/TR/rdf-syntax-grammar/

[RFC2119]

Request for Comments: 2119. Key words for use in RFCs to Indicate Requirement Levels, March 1997 (IETF).
https://www.ietf.org/rfc/rfc2119.txt

[RFC2045]

Request for Comments: 2045. Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies, November 1996 (IETF).
https://www.ietf.org/rfc/rfc2045.txt

[OWL]

Web Ontology Language (OWL) Overview - W3C

[XML]

Extensible Markup Language (XML) 1.0 (Fifth Edition). W3C Recommendation 26 November 2008.
https://www.w3.org/TR/xml/

Appendix E: Contributors

Contributors to this Working Draft: Shadi Abou-Zahra, Philip Ackermann, Carlos Iglesias, Johannes Koch, Michael Squillace, and Carlos Velasco.

Appendix F: Document Changes

The following is a list of substantial changes since the 29 October 2009 Working Draft:

• removed dct:source from Content class
• added dct:hasFormat and dct:isFormatOf to Content class
• removed conformance section
• class and property tables corrected
• schema and schema references updated
• bytes property given a correct datatype
• minor style corrections