Device Independence Principles

W3C Working Group Note 01 September 2003

This version:
Latest version:
Previous version:
Roger Gimson (HP) <roger.gimson@hp.com>
Shlomit Ritz Finkelstein (invited expert) until June 2003
Stéphane Maes (IBM) until July 2002
Lalitha Suryanarayana (SBC Technology Resources) until April 2003
See Acknowledgements


This document celebrates the vision of a device independent Web. It describes device independence principles that can lead towards the achievement of greater device independence for web content and applications.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document has been produced as an overview document from the W3C Device Independence Activity. It is intended to provide a set of principles that can guide work relating to device independence.

This document is one of a series produced by the Device Independence Working Group, part of the W3C Device Independence Activity. The DIWG activity statement can be seen at http://www.w3.org/2001/di/Activity.

A version of this document is also available showing changes since the previous Working Draft.

Comments on, and discussion arising from, this document can be sent to www-di@w3.org, the public forum for discussion of the W3C's work on Device Independence. The archive for the list is accessible online.

Patent disclosures relevant to this document may be found on the WG patent disclosure page.

Table of contents

1 Introduction

1.1 Audience

The intended audience for this document is primarily other W3C Working Groups and external organizations involved in developing web content authoring and delivery technologies.

1.2 Goals

The goal of this document is to suggest that web content and applications can be authored, generated or adapted for a better user experience when delivered via many different web-connectable access mechanisms. The general phrase "device independence" is used for this, although the access mechanisms may include a diversity of devices, user agents, channels, modalities, formats etc.

The vision we share with others is to allow the Web to be accessible by anyone, anywhere, anytime, anyhow. The focus of the W3C Web Accessibility Initiative is on making the Web accessible to anyone, including those with disabilities. The focus of the W3C Internationalization Activity is on making the Web accessible anywhere, including support for many writing systems and languages. The focus of the W3C Device Independence Activity is on making the Web accessible anytime and anyhow, in particular by supporting many access mechanisms (including mobile and personal devices, that can provide access anytime) and many modes of use (including visual and auditory ones, that can provide access anyhow).

These three Activities are complementary, and our interests overlap. For example, being able to select an auditory mode of use on a device may be essential to someone with a visual disability. The focus of device independence is on matching web content to the needs, capabilities and limitations of the delivery environment. We wish to minimize the extent to which web content is authored in a way that is only deliverable on a restricted set of devices.

The principles described here are intended to be complementary to the wider Architecture of the World Wide Web [WEBARCH], which consists of a wider set of requirements, constraints, principles, and choices that influence the design of the Web and the behavior of agents within it. Implementations of these principles should also conform to the Web Content Accessibility Guidelines [WCAG], the User Agent Accessibility Guidelines[UAAG] and the Authoring Tool Accessibility Guidelines [ATAG].

The aim of this document is to focus on device independence and to set out some principles that can be used when evaluating current solutions or proposing new solutions, and can form the basis of more detailed requirements and recommendations.

The principles are independent of any specific markup language, authoring style or adaptation process. They do not propose specific requirements, guidelines or technologies.

It is intended, however, that these principles be used as a foundation when proposing greater device independence through, for example:

Other documents provide more details on the authoring challenges [AC] and the delivery context issues [DCO] associated with achieving device independence.

1.3 Motivation

In recent years, there has been a proliferation of types of device and access mechanisms using the Web, extending far beyond the conventional personal computer. These access mechanisms range from web tablets, appliances and TVs in the home, to mobile devices including phones and PDAs, and access mechanisms for the physically challenged. Connectivity capabilities have also evolved to include high bandwidth modems, LANs and wireless networks. Simultaneously, the needs and expectations of the user with regards to access, availability and consumption of web content have also evolved. Users now expect to get to critical information through different access mechanisms from different locations and at different times during their day.

For example, a user may want to access some web information using a PC connected to a cable network when at home, but when out of the house the same user expects to access the same information using a PDA connected through a mobile phone network.

In the appendix on Usage scenarios we give examples of circumstances in which access to the Web may be required via different access mechanisms.

Content authors can no longer afford to develop content that is targeted for use via a single access mechanism. The key challenge facing them is to enable their content or applications to be delivered through a variety of access mechanisms with a minimum of effort. Implementing a web site or an application with device independence in mind could potentially save costs, and assist the authors in providing users with an improved user experience anytime, anywhere and via any access mechanism.

1.4 Overview

In the following section, the impact of device independence is considered from three different perspectives:

For each perspective, significant concepts for device independence are defined and appear as follows:

Within this document, concept definitions are shown in this style.

A separate Glossary for Device Independence [Glossary] includes all the definitions introduced in the body of this document as well as those from other documents produced by the Device Independence Activity.

Within each perspective, a number of principles are stated. They are intended to capture fundamental assumptions within which specific approaches to authoring and adaptation can be developed.

The principles are not specific to any particular usage scenario or to any implementation technology, but capture aspects of device independence that will apply across many contexts. Each Device Independence Principle (DIP) is given a sequential number and a name. They appear as follows:

DIP-nn: Principle name

Within this document, principles are stated in this style.

Examples are given at certain points within the document in order to illustrate the way in which the principles may be applied in specific circumstances. They appear as follows:

Example: Within this document, examples are shown in this style.

2 Principles

In this section a number of principles are introduced which are fundamental to the achievement of device independence.

They are called principles (rather than, for example, guidelines or requirements) because they attempt to capture important concepts and aspirations that are not specific to any particular realization. Unlike guidelines, they are not specific enough to guide current practice. Guidelines could be developed in due course by applying the principles to the use of current markup technologies. Unlike requirements, they do not propose a specific approach to which some new technology must conform. Requirements for new technology could be developed in due course by applying the principles as part of the design process.

Whether these principles are used to develop future guidelines, requirements and recommendations will depend on their general acceptance within the Web development community. At this stage, all the principles are stated as "should", in lowercase. In the future, if requirements are developed from them, the conventional requirements terminology of "MUST", "SHOULD", and "MAY" could be used to indicate different degrees of conformance (as defined in RFC2119).

For purposes of explanation, the principles are introduced, with the concepts used in their statement, from three perspectives: that of the user, the author and the delivery mechanism. However, the principles themselves can have implications that span multiple parts of the delivery chain.

2.1 User perspective

2.1.1 User-related concepts

The following diagram introduces the key user-related concepts, which are then defined and explained in the subsequent text.

diagram illustrating user concepts

We want to enable the user to perceive and interact with the Web using many kinds of device, or more generally, via many kinds of access mechanism.

The main concepts for the user are as follows.

user experience
a set of material rendered by a user agent which may be perceived by a user and with which interaction may be possible
an apparatus through which a user can perceive and interact with the Web

Example Existing devices that are commonly used to access the Web include PCs, PDAs, web-enabled phones, and interactive TVs.

access mechanism
a combination of hardware (including one or more devices and network connections) and software (including one or more user agents) that allows a user to perceive and interact with the Web using one or more modalities (sight, sound, keyboard, voice etc.)

The access mechanism is an intermediary between the Web and the user. On one side it communicates with the Web using protocols and markup conventions, on the other it supports perception by, and interaction with, the user.

From the user's point of view, an access mechanism often corresponds to a single device.

However, in some situations, the access mechanism may consist of more than one device.

Example A printer may be attached to a PC that is connected to the Web. The combined apparatus provides an access mechanism that can offer a printed representation of a web page.

Example When in a mall, a user may interact with the Web simultaneously through a kiosk interface and a personal phone. Personal information is exchanged through the private phone connected to the mobile network, while public information is viewed, under the control of the phone, on the larger but public kiosk screen equipped with a high-speed connection.

Sometimes the access mechanism may include the use of additional network support.

Example A service provider may offer a service that allows voice interaction with the Web via a telephone. The combination of the telephone and the service is the access mechanism that enables an auditory perception of the web content.

Sometimes the access mechanism may support synchronization for multi-modal interaction.

Example A user may be able to access the Web through a smart phone that simultaneously provides both a graphical interface directly on the phone and a voice interface provided via a network service. The user can select at any time which is the preferred mode of interaction e.g. voice for input and graphics for output, voice for both, or graphics for both.

perceivable unit
a set of material which, when rendered by a user agent, may be perceived by a user and with which interaction may be possible

Most perceivable units provide both presentation and the means for interaction. However, on some types of device, such as printers, perceivable units might contain only presentation.

web page identifier
a Uniform Resource Identifier intended to be recognized by a user as representing the identity of a specific web page (resource)

A user may enter a web page identifier explicitly as part of their interaction, or may indirectly follow one by selecting a link, selecting a previously saved bookmark, or through some other aspect of interacting with an application.

Example A user may see a web page identifier, such as "www.example.com" used in an advertisement. They expect to be able to use this identifier to access further information about the advertiser or product in the form of an appropriate user experience.

In some cases, such as in voice-only browsing, the user may not know the web page identifier being used to retrieve the next perceivable unit.

A perceivable unit is rendered to the user as a collection of physical effects, visual, auditory or tactile, via one or more devices within the access mechanism. Mechanical controls, such as buttons, keys and pointers, and sonic input such as voice commands (and possibly other affordances) allow a user to interact with the perceivable unit.

Example If a user entered the web page identifier "www.example.com" into a web-connected PDA, the resultant perceivable unit may be a visual display showing the company logo, a brief description of its activities, and a link that when touched leads to another page with additional product information.

The physical user interface is where the information that was transmitted digitally becomes accessible to and manipulable by the user.

web page
a collection of information, consisting of one or more resources intended to be rendered simultaneously, and identified by a single Uniform Resource Identifier

Conventionally, a web page may be thought of as a perceivable unit. However, depending on the access mechanism, and the granularity of its presentation capabilities, the number of perceivable units required to present a web page may vary.

Example A limited capability device, such as a mobile phone, may accept only small amounts of data during a single interaction with the Web. Such a device may be unable to accept a large resource in a single perceivable unit. The limited presentation and input capabilities on this kind of device may also favor use of multiple perceivable units for improved usability.

The quality of the user experience depends on the degree to which material has been successfully adapted for use via a specific access mechanism. The possible modes of interaction are largely defined by the device manufacturer, but the way in which they are incorporated into one or more perceivable units will affect the ease of use of that device for the user.

The next concept is about the quality of the user experience delivered via a given access mechanism in response to a given web page identifier. It is intended to capture the minimal quality of user experience necessary to be useful to a user.

functional user experience
a set of one or more perceivable units that enables a user to complete the function intended by the author for a given resource via a given access mechanism

The function intended by the author may be as simple as to convey some information to the user.

Example If the intended function is for the user to be able to perceive a passage of text, a functional user experience might provide visible or audible representations of it.

The function intended may be as complex as guiding the user through a sequence of application-specific interactions.

Example If the intended function is for the user to take the next step in some interactive application, a functional user experience should be sufficient for the user, say, to successfully fill in a data field and submit it.

A functional user experience is defined with respect to a particular access mechanism. It may not be possible to achieve a functional user experience for some particular content or application for every access mechanism, due to inherent limitations of the mechanism.

The functional intent is defined by the author of the application, and it is possible that it will differ from the expectation or intent of the user.

Example A user may access a flight reservation system in order to find the abbreviated code used for a particular airport. If the user accesses the flight reservation system by voice, the abbreviated code may not be provided, since the airport name may only be spoken without abbreviation. The user may consider that this particular audible representation is not functional for his or her purpose. However, this is a user-centric definition of functional user experience. The user experience is functional from the author's perspective. However the application does not deliver the information to the user in the form they desire.

Whether the user experience associated with a web page is considered by the user to be functional clearly depends on their understanding of the function associated with that web page. The user may already have an expectation of the functionality, for example because they have previously accessed the page on another device. If their expectations are not met, the user cannot be certain whether it is because their expectations are wrong or whether a non-functional user experience has been delivered. If it is the first time they have accessed the web page, they can only rely upon the integrity and consistency of the user experience to estimate whether it is functional.

Many things may prevent completion of a function, such as when a user experience of a web page is not made available, or has broken elements, or causes a failure in the access mechanism. From a user's perspective, if the user is unable to carry out the function successfully then the user experience is not functional.

2.1.2 User-related principles

DIP-1: Device independent access

For some web content or application to be device independent, it should be possible for a user to obtain a functional user experience associated with its web page identifier via any access mechanism.

This is the fundamental principle for device independence from the user perspective. It does not say that the user experience will be the same on every device. But it does say that the user should be able to obtain a user experience and that the user experience obtained should be at least functional.

Example A user enters a web page identifier for a weather forecasting page on different devices. On the screen of a PDA, the text "Tomorrow" is shown, together with a graphic of a sun. On the screen of a phone, the text "Tomorrow: sun" is shown. On an in-car PC, the words "Tomorrow will be sunny" are spoken. These are all user experiences that have been adapted to be functional for their specific access mechanisms.

Example If on some device the same web page identifier as in the previous example gave either no user experience, or a user experience consisting of the text "Cannot display graphics", this would not be a functional user experience.

The goal is that a functional user experience should be possible via any access mechanism. The method by which presentation and interaction are provided may vary according to the different access mechanisms, but the possibility of a functional user experience should always exist.

In particular, it should be possible to provide a functional user experience even on a limited capability device - though it may be of reduced quality compared to that on more capable devices.

Example Where an image would occur on an image-capable device, a text alternative could be displayed on a text-only device, or the text could be spoken when accessed by a voice-only phone.

DIP-2: Device independent Web page identifiers

A web page identifier that provides a functional user experience via one access mechanism should also provide a user experience of equivalent functionality via any other access mechanism.

In other words, the intended function of a web page is associated with the web page identifier, and should apply to all user experiences obtained from the web page identifier, no matter what the access mechanism.

Example By bookmarking a web page identifier that provides a functional user experience on one device it should be possible to obtain a corresponding user experience on another device with equivalent functionality. The user experiences may be different due, among other things, to the different capabilities of the devices, but their intended function is the same and they should both be at least functional.

In order to meet this principle, it may be necessary to restrict or re-interpret what is acceptable as a web page identifier. For example, a URI that includes a suffix indicating its representation, such as "example.html", may need the suffix to be ignored when the material is adapted for a non-HTML device. Similarly, any device-specific information coded in a query part or fragment part of the URI may need to be re-interpreted or ignored. Unfortunately, many implementers do not respect the intent of URI style guidelines [URI-Style].

2.2 Authoring perspective

2.2.1 Authoring-related concepts

The following diagram introduces the key authoring concepts, which are then defined and explained in the subsequent text.

diagram illustrating authoring concepts

The main concepts for authoring are as follows.

When a request is made over the Web for a web page, not only should the request specify the web page identifier for the page, but also it should provide enough information about the access mechanism and the user that the right kind of user experience can be provided.

delivery context
a set of attributes that characterizes the capabilities of the access mechanism and the preferences of the user

Delivery context information may be sent as part of each request, but this is not essential. Alternative possible implementations are discussed in the separate Delivery Context Overview [DCO] document.

The delivery context expresses the capabilities and preferences that may constrain the acceptable range of user experiences that can be delivered via a given access mechanism. In particular, the capabilities of the device, including the modalities and representations it supports, the characteristics of the network over which delivery occurs, and the preferences of the user will all potentially affect the user experience provided.

Example A PDA may have a screen of a certain size, and have a built-in speaker. It may be able to display HTML markup and play streaming audio. It may be connected to the Web over a phone network that only offers a slow data transfer rate. The user of the PDA may be in a meeting and prefer not to use audio at that moment. These (and other) attributes form the current delivery context. In this delivery context it might be appropriate to present a web page as a limited amount of text on the screen.

delivery unit
a set of material transferred between two cooperating web programs as the response to a single HTTP request

The transfer might, for example, be between an origin server and a user agent. Users are not normally aware of individual delivery units.

The response to a request containing a web page identifier is a delivery unit which contains the data (in a markup language or image encoding, for example) that will be used by the access mechanism in order to render a physical user experience. It will also allow the user to interact with that page, and possibly generate a further request.

Example Markup languages such as HTML, XHTML, SVG, etc. are designed specifically as ways of representing aspects of user experiences. Image formats such as PNG and scripting languages such as ECMAScript may also be used to represent different aspects of the user experience. It is common to find certain combinations of representations that are delivered together. For example, HTML pages frequently contain ECMAScript. For such combinations, a single delivery unit can carry multiple representations. Some delivery units, especially those associated with media, often carry only a single representation. Images are an example of media often carried within their own delivery units.

a process of selection, generation or modification that produces one or more perceivable units in response to a requested uniform resource identifier in a given delivery context

Adaptation is shown in this diagram as occurring at the server (and hence known as server-side adaptation). However, it may also occur at intermediate points in the delivery chain, or at the client (known as client-side adaptation). See the Delivery Context Overview [DCO] for further explanation.

There are many techniques that could be used to produce an appropriate user experience in response to a request for a web page. Since these principles are intended to be technology independent, no further exploration of the adaptation process is made here.

The authoring perspective instead focuses on the quality of user experience produced by the adaptation process and on the obligations on the authoring task.

The concept of functional user experience has already been introduced as part of the user perspective.

However, though a user experience may be functional, in that it is sufficient to allow a user to complete the function intended, it may not provide a rendering that matches the author's objective for a particular delivery context. The next concept sets out to address this.

harmonized user experience
a functional user experience that is sufficiently harmonized with the delivery context to meet the quality criteria of the author

Harmonization allows the user experience to be made more specific to the delivery context in order to meet criteria, such as ease of use and quality of user experience, that are important to the author. They may affect, perhaps, the popularity of the web page or the trust that the user places in the underlying application.

Example The way that information is laid out, in the visual representation of a web page, may take into account the dimensions of the display of the device being used. This information is made available to the adaptation via the delivery context. The same information may be arranged quite differently on the same device to improve the visual effect or to improve the usability of the user interaction. Such changes do not affect the functionality of the user experience but may improve its appeal. These aspects are often important to authors and to users.

Authoring tools and adaptation processes may differ in the extent to which they support harmonized user experiences.

2.2.2 Authoring-related principles

The following principles are directed towards the authoring and adaptation process that is responsible for providing a delivery unit in reply to a request from a user.

DIP-3: Functionality

It should be possible to provide a functional user experience, in response to a request for a web page, in any given delivery context that has an adequate access mechanism.

This principle places an obligation on the author, the authoring tools and the adaptation process to provide a user experience that allows a user to successfully access a web page to get the information or complete the interaction intended.

The principle is effectively a restatement of DIP-1 from the point of view of the producer rather than the consumer.

Sometimes it is not possible to provide some content or functionality in a given delivery context because of limitations in the access mechanism.

DIP-4: Incompatible access mechanism

If a functional user experience of an application cannot be provided due to inherent limitations in the access mechanism, an explanatory message should be provided to the user.

Example An access mechanism has no sound output capabilities. An application wishes to offer a function, such as playing some music, that can only be provided through sound output. The application cannot offer a functional user experience through such an access mechanism. In this case, it should provide a message to the user instead that indicates the nature of the incompatibility (and might suggest alternatives).

DIP-5: Harmonization

If the author wishes, it should be possible to provide a harmonized user experience, in response to a request for a web page, in any given delivery context that has an adequate access mechanism.

This goes beyond the previous principle to additionally allow that the author can provide a user experience that is well designed for the delivery context. It is an obligation on the authoring tools and adaptation process to support the creation and delivery of user experiences tailored for specific access mechanisms when the author requires them.

In many situations, it is necessary to define specific details about the user experience associated with a particular web page. It is not sufficient that only predefined or standard user experiences can be used. Authors should be able to control the precise details of the user experience if they wish. At the most detailed level, a harmonized user experience might apply to only a very restricted set of delivery contexts. However, DIP-2 requires that other adaptations be available to provide functional user experiences for the other delivery contexts.

Example Some devices, especially small ones such as phones, are highly constrained in their capabilities. To create a sufficiently user-friendly user experience of some application, it may be necessary to harmonize the user experience for that particular application on that particular phone.

2.2.3 Device independent authoring

It is unrealistic to expect an author to create different user experiences for every delivery context. Whenever possible, authored source content should be reused across multiple delivery contexts.

Functional user experiences suited to any delivery context can be generated by using an adaptation process applied to a representation that does not depend on the access mechanism.

Example One approach to device independent authoring is to use a transformation technique such as XSLT. By creating the content in XML, the same content can be adapted for different delivery contexts using XSL stylesheets.

Some authoring approaches based on device independent representations may in addition allow customized user experiences to be provided for some delivery contexts.

The issues facing authors are described in more detail in the separate document Authoring Challenges for Device Independence [AC].

2.3 Delivery perspective

2.3.1 Delivery-related concepts

The following diagram introduces the key delivery concepts, which are then defined and explained in the subsequent text.

diagram illustrating delivery concepts

The main concepts for delivery are as follows.

user agent
a client within a device that performs rendering
the act of converting perceivable units into physical effects that can be perceivable by a user and with which the user may be able to interact

The user agent is some software or firmware in a device (for example a browser) that lets the user identify a web page to be rendered, assembles an appropriate request for that page (possibly including the delivery context), accepts the reply (delivery unit), and renders that reply into one or more perceivable units.

Before rendering, a user agent may apply client-side adaptation, perhaps under the control of adaptation preferences, to transform a delivery unit into a perceivable unit.

Example A user agent may render the material they receive in a delivery unit as a single perceivable unit or as multiple perceivable units, perhaps to better fit a limited viewing area. For example, WML provides explicit markup for the delivery unit, <deck>, and the perceivable unit, <card>.

There are three distinct ways in which the user may personalize how a web page is presented to them. They are distinguished by which parts of the delivery chain are responsible for affecting the user experience: the renderer in the user agent, some adaptation process (in the server, client or an intermediary), or the functionality of the application itself.

rendering preferences
a set of preferences, specified by a user, that may affect the way the user agent renders a perceivable unit, and so change the resultant user experience

It may be possible for a user agent, under the control of rendering preferences, to make local changes to a perceivable unit as it is rendered. This will depend on the local behavior of the user agent within the device.

Example A user agent may allow the user to locally increase the size of text, or convert text to speech, to improve accessibility for those with poor or no sight.

In current user agents, such rendering preferences are not communicated as part of the delivery context and so do not affect the delivery unit sent to the device. However, this also means it is possible for a user experience to become non-functional through inappropriate choice of rendering preferences.

Example A user experience may use black text on a white background. If a local rendering preference sets only the background to be black, the text will become unreadable and so non-functional.

adaptation preferences
a set of preferences, specified by a user, that may affect the adaptation for a given delivery context, and so change the resultant user experience

The user may be able to express preferences that affect the way the content or application functionality is presented when there is more than one choice. Some devices can support more than one way of presenting a web page, or more than one method of interaction with it.

Example Even if an access mechanism supports images, a user may prefer to receive all information as text - perhaps because they are using a local text-to-speech converter. By indicating an adaptation preference for no images, the adaptation process may be able to provide a better text-only presentation, harmonized with the delivery context, than could be achieved by locally replacing all images by their text alternates.

Some adaptation may occur within the network, for example in transcoding proxies, or in the client prior to the final perceivable unit being rendered.

Example A web page may be delivered as some HTML markup with embedded CSS styles that depend on media types. The identification of the appropriate media type according to the delivery context, and the resultant selection of the style to apply, should be considered as part of the client-side adaptation process. The result of selecting the appropriate style is a perceivable unit that can be rendered into physical effects. The application of a CSS style sheet according to a media type should therefore be considered as part of the adaptation process under the control of adaptation preferences.

Some user experience changes may be achieved either through rendering preferences or through adaptation preferences. However, as illustrated in Example, it may be better to use adaptation preferences, if available, to avoid the possibility of breaking user experience functionality.

application personalization
a set of factors, specified by a user or other aspects of the delivery context, that may affect the functionality of an application, independently of its adaptation and delivery, and so change the resultant user experience

Application personalization may include, for example, the preferred language or the preferred location to which the user experience should if possible be made specific.

Differences in user experience that are the result of application personalization are not considered within the scope of device independence.

Example A weather-forecasting application may provide a forecast that is selected for the locality of the user. The fact that the forecast is for a specific location is part of the application, though it may use location information taken from the delivery context. Localization is a form of application personalization, and is a separate issue from the way the resultant forecast is presented to the user - for example as a visual icon or as a text message - which can be affected by adaptation preferences.

2.3.2 Delivery-related principles

DIP-6: Characterization of delivery context

The user agent should be able to associate the characteristics of the delivery context with a request for a particular web page.

Unless the characteristics of the delivery context can be made available to the adaptation process, it will not be possible to know whether a specific user experience of some web page can be delivered in that context, or how to generate a suitable user experience.

Example If it is known that only a monochrome screen is available, different attributes than color may be used to highlight important passages of text.

Example If it is known that the delivery is to take place over a slow network connection, images may be reduced in size or converted to text alternatives.

DIP-7: Adaptation preferences

It should be possible for a user to provide or update any adaptation preferences as part of the delivery context.

It is not only the user agent that may automatically contribute to the adaptation preferences by providing characteristics of the delivery context. The user may also provide adaptation preferences that may be used by the adaptation process to offer a more suitable user experience, after taking into account the constraints of the network and device. The adaptation process may allow a user to obtain the most appropriate user experience for their abilities and circumstances.

Example If in a hurry, a user may request a lower quality presentation, such as lower resolution images, in order to speed up web page delivery.

Example If in a car, a user may switch between a visual presentation and a voice-only presentation, depending on whether they are stationary or driving.

An author may wish to provide a specific user experience that is harmonized with the access mechanism in a way that they define. But it is important that this does not result in excluding users from obtaining a functional experience that is best suited to their needs. Guideline 2 of the User Agent Accessibility Guidelines [UAAG] states that users must have access to all content, including conditional content. For example, the user agent could allow the user to update a description of the delivery context to select the most appropriate adapted version of the content for their needs.

3 Associated work

As indicated in the Introduction, these principles can form the foundation of many approaches to achieving greater device independence: the creation of guidelines, modification or extensions to existing markup languages, adaptation techniques or new markup languages.

The W3C Technical Architecture Group is currently documenting principles of Web architecture [WEBARCH], which include issues of presentation, content and interaction that are relevant to delivery across a range of devices.

The W3C Device Independence Working Group is currently chartered [DIWG] to develop requirements for device independence and to review device independence issues that may arise in other W3C groups and external bodies.

In this section, we provide some pointers to associated documents and current work at the time of writing. Please refer to the W3C Device Independence Activity home page for the latest information.

The issues faced by authors in creating web pages that can be delivered to many kinds of device are highlighted in Authoring Challenges for Device Independence [AC]. Current work is in progress on providing an associated document on Authoring Techniques for Device Independence.

The issues of representing and conveying delivery context information are separately considered in Delivery Context Overview for Device Independence [DCO]. Current work is in progress on defining Core Presentation Characteristics [CPC] that could be used to specify device capabilities within the delivery context. Also, work on Composite Capabilities/Preferences Profile [CCPP] will allow delivery context information to be represented and conveyed as part of a request for a web page.

4 References

Authoring Challenges for Device Independence, W3C Working Group Note 01 September 2003
Authoring Tool Accessibility Guidelines 2.0, W3C Working Draft 14 March 2003
For latest version see:http://www.w3.org/TR/ATAG20/
Composite Capability/Preference Profiles (CC/PP): Structure and Vocabularies, W3C Working Draft 28 July 2003
For latest version see: http://www.w3.org/TR/CCPP-struct-vocab/
Core Presentation Characteristics: Requirements and Use Cases, W3C Working Draft 10 May 2003
For latest version see: http://www.w3.org/TR/cpc-req/
Delivery Context Overview for Device Independence, W3C Working Draft 13 December 2002
For latest version see: http://www.w3.org/TR/di-dco/
W3C Workshop on Web Device Independent Authoring, Bristol UK 3-4 October 2000
W3C Workshop on Device Independent Authoring Techniques, St. Leon-Rot, Germany 25-26 September 2002
Device Independence Working Group Charter, W3C Device Independence Activity 12 June 2002
W3C Delivery Context Workshop, St. Leon-Rot, Germany 25-26 September 2002
Glossary for Device Independence, W3C Working Draft 25 August 2003
For latest version see: http://www.w3.org/TR/di-gloss/
W3C/WAP Workshop on the Multimodal Web, Hong Kong 5-6 September 2000
How People with Disabilities use the Web, W3C Working Draft 4 January 2001
For latest version see: http://www.w3.org/WAI/EO/Drafts/PWD-Use-Web/
User Agent Accessibility Guidelines 1.0, W3C Recommendation 17 December 2002
Cool URIs don't change, Tim Berners-Lee 1998
Web Content Accessibility Guidelines 2.0, W3C Working Draft 24 June 2003
For latest version see: http://www.w3.org/TR/WCAG20/
Architecture of the World Wide Web, W3C Working Draft 27 June 2003
For latest version see: http://www.w3.org/TR/webarch/


A Usage scenarios

This section illustrates various ways in which users interact with the Web. The examples outlined here are intended to portray the potential benefits of device independence from an end user perspective. Associated with each scenario is a determination of the relevance to device independence and the principles outlined in the text above, and the considerations from an authoring perspective.

A.1 Choice of access device

Ms. Kaseem is a teenager with low vision who is also deaf. She uses the Web to find new restaurants to go to with her friends and classmates. At home she uses a combination of screen magnifier and screen reader with refreshable Braille to browse the Web. She also has a portable Braille device which she uses in public places such as malls and restaurants. See the teenager example from How People with Disabilities Use the Web [PWD] for a detailed outline of the use case.

Relevance to Device Independence:: Depending upon her needs, Ms. Kaseem uses different access mechanisms to interact with the Web. Accessibility may be improved through device independence.

Authoring perspective: The web page should be accessible to Ms. Kaseem in terms of support for functional user experiences on multiple devices.

A.2 Choice of connectivity

Peter is a sales Manager for a Silicon Valley company, responsible for all accounts in the US Midwest. While he is not on the road, he works out of his Denver home. He accesses the web based corporate Sales Force Automation application from his laptop PC over a DSL 1.5 Mbps connection in his home office. However when he is at customer sites, he uses a PCMCIA-based GPRS modem in his laptop to get to the application over a wireless network.

Relevance to Device Independence: A change in the delivery context resulting from a choice of available connection transport mechanisms to access the Web is within the scope of device independence.

Authoring perspective: The presentation should vary according to connectivity capabilities. The functional user experience under either mechanism could likely be the same, but the harmonized user experience would probably be tailored to the characteristics of the underlying network. For example, rich content such as high resolution images would be possible over a DSL line, but the same image could have relatively lower resolution to speed up download over a 28.8 kbps connection.

A.3 Choice of interaction modality

Nancy has a 45 minute work commute across town every day. Just as she is stepping out, she checks traffic conditions from her favorite local web site using her web enabled mobile phone. Once she gets into the car, the car computer (with position sensing capability) synchronizes with her phone. Today she is stuck in traffic midway. She uses a hands-free voice interface in order to discover alternate routes that will get her to the office faster.

Relevance to Device Independence: The modality of interaction that Nancy uses to obtain information from the Web has changed depending upon whether she is driving or stationary. Modality choices should be supported by device independence.

Authoring perspective: Modality is part of the delivery context and should be taken into consideration while determining the user experience provided by a web page.

A.4 Choice of presentation

Mr. Wright has purchased pre-paid GPRS enabled PDAs for his sons, Jimmy and Tommy. While they both like to download the latest Dennis the Menace cartoon from the same web site, Jimmy and Tommy however have different views on how to spend the $30 pre-paid card their dad has given them. Jimmy prefers the cartoon to be downloaded as high resolution images most of the time, while Tommy prefers a small picture in low resolution graphics, thus saving up to chat with his friends via instant messages later.

Relevance to Device Independence: The preferences of the user are part of the delivery context and may impact the resultant user experience. This is within the scope of the principles for device independence.

Authoring perspective: The author may wish to support multiple user experiences of the same web page depending upon individual adaptation preferences. Such preferences are independent of the rendering preferences that may be provided locally in the user agent.

A.5 Multi-modal interaction

Mr. Gray is an executive who travels to headquarters regularly for staff meetings. Today the meeting is running later than usual. He needs to rearrange his schedule such that he is on a later flight in the evening. He uses his web enabled phone to reschedule his flights. He enters his flight preferences by talking to the system, but selects seating via graphical display. The confirmation is played to him by voice, but he also receives a text notification which he can save for later reference.

Relevance to Device Independence: Device independence is concerned about the ability to provide user experiences for different modalities, including multi-modal access mechanisms.

Authoring perspective: The author may need to take into account the multiple modalities active during a navigation session and their interactions in such a way as to provide a functional or customized user experience to the user. However, the special considerations involved in synchronizing multi-modal presentations are outside the scope of the Device Independence Working Group.

A.6 Transactions that span devices

Angela subscribes to a service run by a local concert hall that informs her of late availability of tickets at discount prices. At work one morning, she receives notification of availability of tickets for a concert including Mahler's Fifth Symphony. She has been waiting for the opportunity to take her mother to a Mahler concert for some time. Using her PC she provisionally books two tickets in prime locations in the knowledge that she must purchase them within 6 hours or lose the booking. She bookmarks the acknowledgement returned by the concert hall and synchronizes it with her PDA. Later in the day, on the way back to the office after a meeting with a client, she finally manages to contact her mother, who is delighted at the invitation and looking forward to the concert. Angela realizes that she must confirm the booking soon. She returns to the concert hall's bookmark using her PDA, confirms the booking and pays for the tickets.

Relevance to Device Independence: The concert hall web site is enabled for access from multiple devices. Angela needs to bookmark only one URI for her transaction, regardless of the device she accesses the site from. This is within the scope of device independence.

Authoring perspective: A web site that is user friendly provides device independent URIs for access to its services (including any encoding of transaction identity). The transaction may be initiated using one access mechanism and consummated via another, seamlessly, even though the user experience might vary.

B Main changes since last working draft

Since the last Working Draft of this document, some terminology has been changed.

'User experience' is now used instead of 'presentation', so that it is clearer that user interaction (input) as well as rendered effects (output) are included in the term.

'Harmonized' is now used instead of 'customized', so that there is less likely to be confusion with application-specific customization. For example, a web page that presents application information that is specific to a particular context, such as a weather forecast for a locality, may be called 'customized', whereas 'harmonized' is now used to mean that the user experience of perceiving that information has been well-matched to the access mechanism.

'Adaptation preferences' is now used instead of 'presentation preferences' to make the distinction from rendering preferences and application customization clearer.

The 'Glossary' Appendix has been removed and replaced by references to a separate Glossary document.

The 'Implementation scenarios' Appendix has also been removed pending a future, more comprehensive, survey of authoring techniques.

Some changes have also been made in response to the issues raised on the last Working Draft.

C Acknowledgements

Members of the W3C Device Independence Working Group have helped develop this Working Group Note through their comments, proposals and discussions at teleconferences, face-to-face meetings and via the group discussion list.

At the time of publication, the principal and active members of the group were as follows:

Stephane Boyera (W3C)
Steve Farowich (Boeing)
Roger Gimson (HP)
Yoshihisa Gonno (Sony Corp)
Guido Grassel (Nokia)
Rotan Hanrahan (MobileAware Ltd)
Kazuhiro Kitagawa (W3C)
Markus Lauff (SAP AG)
Tayeb Lemlouma (INRIA)
Rhys Lewis (Volantis Systems Ltd)
Roland Merrick (IBM)
Franklin Reynolds (Nokia)
Andreas Schade (IBM)
Ryuji Tamagawa (Sky Think System)
Luu Tran (Sun Microsystems)
Michael Wasmund (IBM)
Stan Wiechers (Merkwelt)
Jason White (University of Melbourne)
Candy Wong (NTT DoCoMo)
Amy Yu (SAP AG)

The following were members of the group at earlier stages of its drafting:

Yasser AlSafadi (Philips Research)
Abbie Barbir (Nortel Networks)
Einar Breen (Adaptive Media)
Shlomit Ritz Finkelstein (invited expert)
Vidhya Golkar (Argogroup)
Luo Haiping (Comverse)
Eric Hsi (Philips Research)
Lynda Jones (SHARE)
William Loughborough (Smith-Kettlewell Institute)
Stephane Maes (IBM)
Kaori Nakai (NTT DoCoMo)
Hidetaka Ohto (W3C/Panasonic)
Garland Phillips (Motorola)
Lalitha Suryanarayana (SBC Technology Resources)
Yoshifumi Yonemoto (NTT DoCoMo)