Mobile Web Application Best Practices

Editors' Draft 13th March 2009

This version:
Latest version:
Previous version:
http://www.w3.org/2005/MWI/BPWG/Group/Drafts/BestPractices-2.0/ED-mobile-bp2-20090101 (diff)
Bryan Sullivan, AT&T
Adam Connors, Google


This document specifies Best Practices for the development and delivery of Web applications on mobile devices. The recommendations expand upon statements made in the Mobile Web Best Practices 1.0 (BP1), especially concerning statements that relate to the exploitation of device capabilities and awareness of the delivery context. Furthermore, since BP1 was written, networks and devices have continued to evolve, with the result that a number of Best Practices that were omitted from BP1 can now be included.

The recommendation is primarily directed at creators, maintainers and operators of Web applications. Readers of this document are expected to be familiar with the creation of Web sites, and to have a general familiarity with the technologies involved, such as Web servers, HTTP, and Web application technologies. Readers are not expected to have a background in mobile technologies or previous experience with BP1.

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/.

Incomplete draft: This document is an editors' copy that has no official standing. It is subject to major changes and is therefore not intended for implementation. In particular, the list of Best Practices is not settled yet. The document is provided for review and feedback only. Please send feedback to public-bpwg-comments@w3.org (archive).

The Working Group is particularly seeking feedback on:

This document was developed by the Mobile Web Best Practices Working Group as part of the Mobile Web Initiative. Most of the best practices statements were changed since the publication of the First Public Working Draft on 29 July 2008. A complete list of changes is available.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document is a Second Public Working Draft intended to progress along the Recommendation track and be eventually published as a Recommendation.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. This document is informative only. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of Contents

List of Best Practices
1 Introduction
    1.1 Purpose of the Document
    1.2 Audience
    1.3 Scope
        1.3.1 Best Practices
        1.3.2 Web Application
        1.3.3 Mobile Context
        1.3.4 Delivery Context
    1.4 Relationship to other Best Practices and recommendations
    1.5 Terminology
2 Structure of Best Practice Statements
3 Best Practice Statements
    3.1 Application Data
        3.1.1 Use Cookies for Simple Client-Side State
        3.1.2 Use HTML5 Client-Side Storage for Local Data Model
        3.1.3 Replicate Local State Changes to a Server When Possible
    3.2 Security and privacy
        3.2.1 Do not Execute Untrusted JavaScript
    3.3 User Awareness and Control
        3.3.1 Inform the User About Automatic Network Access
        3.3.2 Provide Sufficient Means to Control Automatic Network Access
        3.3.3 Ensure the User is Informed About Use of Personal and Device Information
    3.4 Conservative use of resources
        3.4.1 Use Transfer Compression
        3.4.2 Minimize Application Size
        3.4.3 Avoid Redirects
        3.4.4 Optimize Network Requests
        3.4.5 Minimize External Resources
        3.4.6 Consider Including Small Style Sheets and Script Resources In-Line
        3.4.7 Aggregate Static Images ("Sprites") into a Single Composite Resource
        3.4.8 Include Background Images Inline in CSS Style Sheets
        3.4.9 Use Fingerprinting to Cache Dynamic Resources
        3.4.10 Cache AJAX Data
        3.4.11 Use Power-Efficient Methods
        3.4.12 Minimize DOM Manipulation
        3.4.13 Reduce Cookie Size
        3.4.14 Don't Send Cookie Information Unnecessarily
    3.5 User Experience
        3.5.1 Design for Multiple Interaction Methods
        3.5.2 Use Scripting to Improve Perceived Performance
        3.5.3 Preserve Focus on Dynamic Page Updates
        3.5.4 Group Closely Coupled Views
        3.5.5 Use Fragment IDs to Maintain Application History
        3.5.6 Make Telephone Numbers "Click-to-Call"
        3.5.7 Ensure Paragraph Text Flows
        3.5.8 Separate Rarely Used Functionality
        3.5.9 Enable Progressive Rendering
        3.5.10 Ensure Consistency Between Desktop and Mobile
        3.5.11 Consider Using Push
        3.5.12 Use Canvas Tag For Dynamic Graphics
        3.5.13 Use viewport Meta Tag To Identify Desired Screen Size
    3.6 Handling Device Capability Variation
        3.6.1 Use Server-side Capability Detection
        3.6.2 Use Client-side Capability Detection for Dynamic Device State
        3.6.3 Use Device Classification to Simplify Content Adaptation
        3.6.4 Support a non-JavaScript Variant if Possible
        3.6.5 Offer Users a Choice of Interfaces


Best Practice Dependent Device Properties
A Sources (Non-Normative)
B Related Reading (Non-Normative)
C Acknowledgements (Non-Normative)
D References (Non-Normative)
    D.1 MWI References
    D.2 Sources
    D.3 Device Independence
    D.4 Web, Protocols and Languages

List of Best Practices

The following Best Practices are discussed in this document and listed here for convenience.


1 Introduction

1.1 Purpose of the Document

This document sets out a series of recommendations designed to facilitate development and delivery of Web applications on mobile devices. The recommendations are offered to creators, maintainers and operators of mobile Web sites.

1.2 Audience

Readers of this document are expected to be familiar with the creation of Web applications, and to have a general familiarity with the technologies involved, but are not expected to have a background in mobile technologies or previous experience with Mobile Web Best Practices 1.0 (BP1).

The intention is to make clear to all involved what the Best Practices are, and hence establish a common basis of understanding. As a result of wishing to be clear to those not already involved in the development of mobile friendly content, some statements may appear to be obvious or trivial to those with experience in this area.

The document is not targeted solely at developers; others, such as interaction and graphic designers, and tool developers, are encouraged to read it.

1.3 Scope

These recommendations expand on the recommendations of the Mobile Web Best Practices (BP1) [ref]. Where the focus of BP1 is primarily the extension of Web browsing to mobile devices, this document considers the development of Web applications on mobile devices.

1.3.1 Best Practices

The approach in writing this document has been to collate and present the most relevant engineering practices prevalent in the development community today and identify those that: a) facilitate the exploitation of device capabilities to enable a better user experience; or b) are considered harmful and can have non-obvious detrimental effects on the overall quality of the application.

The goal of this document is not to invent or endorse future technologies. However, there are a number of cases where explicitly omitting a Best Practice that referred to an emerging technology on the grounds that it was too recent to have received wide adoption would have unnecessarily excluded a valuable recommendation. As such, some Best Practices have been included on the grounds that the Working Group believes that they will soon become fully qualified Best Practices (e.g. in prevalent use within the development community).

1.3.2 Web Application

For the purposes of this document, the term "Web application" refers to a Web page (XHTML or a variant thereof + CSS) or collection of Web pages delivered over HTTP which use either server-side or client-side processing (e.g. JavaScript) to provide an "application-like" experience within a Web browser. Web applications are distinct from simple Web content (the focus of BP1) in that they include some elements of interactivity and persistent state.

While the focus of this document is to document Best Practices that apply to applications running in a Web browser, in many cases these recommendations are equally applicable to other kinds of Web run-time, such as the widget frameworks being considered as part of the Web Widgets [REF] [@@ref needed] effort and also in a number of vendor-specific initiatives.

1.3.3 Mobile Context

In an increasingly mobilized world the line between mobile and non-mobile is necessarily blurred and a document that restricts its focus solely to best practices that are uniquely mobile would most likely be very short. With this in mind, the focus of this document is to address those aspects of Web application development for which there are additional, non-trivial concerns associated with the mobile context. This applies equally both to the limitations of the mobile context (e.g. small screen, poor connectivity), and also the additional scope and features that must be considered when developing for the mobile context (e.g. device context / location, presence of personal data on the device, etc).

1.3.4 Delivery Context

Requirements on delivery context have not been made explicitly, but most best practices assume devices with basic XHTML, JavaScript, and CSS compliance. Additionally, some best practices are relevant only if the device exposes certain capabilities (for example, access to device information such as location).

Implied by this discussion is that some level of device knowledge and content adaptation is required. For best practices specifically related to this area, see 3.6.1 Use Server-side Capability Detection for Device Properties and 3.6.2 Use Client-side Capability Detection for Device State.

1.4 Relationship to other Best Practices and recommendations

These recommendations are complimentary to the recommendations of Mobile Web Best Practices 1.0 (BP1).

This document builds on some of the concepts described by the Ubiquitous Web Applications Working Group (UWA) and the Device Independence Principles [DIP]. The document discusses device and delivery channel characteristics, which the UWA has named "Delivery Context" [DCODI]. In addition, the document uses some terminology from UWA's Glossary of Terms for Device Independence [DIGLOSS].

1.5 Terminology

Note that the term "JavaScript" is used in place of the (arguably more correct) term "ECMAScript" in order to provide consistency with the companion Web application technologies (JSON and AJAX) which are in common use and both implicitly refer to JavaScript in their names.

Also, the terms "AJAX" and XMLHttpRequest (XHR) are used to refer to any asynchronous browser request. The implicit reference to XML suggested by the names is commonly accepted to be an historical anomaly.

2 Structure of Best Practice Statements

The Heading
A summary of the functional area to be addressed by these statements
What it means
An explanation of the intention of the Best Practice statement.
How to do it
A discussion of the techniques and device capabilities required to implement this Best Practice.
A summary of device capabilities required in order for this Best Practice to apply.

3 Best Practice Statements

3.1 Application Data

Most applications have the need to store data of various forms, both intrinsic content (e.g. the emails of an email application, the calendar events of a calendar application) and supplementary personalization settings (e.g. preferred theme, default view, etc).

These Best Practices relate to the appropriate technologies and techniques to use for managing a Web application's data model.

3.1.1 Use Cookies for Simple Client-Side State What it means

Cookies are a common and effective means to store small amounts of state on the client. They are most appropriate for simple personalization data and are also commonly used to store a token representing user identity in order to enable automatic sign-in.

Note however that cookie support cannot be relied upon and may be disabled either in the device configuration or by the mobile network. For this reason applications should endeavour to remain as functional as possible even if cookies are unavailable.

Also note that cookie data is sent to the server with each request and so an excessive volume of cookie data can degrade performance over mobile networks.

See 3.4.14 Reduce Cookie Size and BP1 [COOKIES] Do not rely on cookies being available for appropriate cookie related caveats.

3.1.2 Use HTML5 Client-Side Storage for Local Data Model What it means

If HTML5 is supported, client-side storage APIs can be used to store data locally on the device. For a more detailed technical summary see Offline Web Applications.

Client-side storage provides a powerful mechanism for Web applications to maintain a local data model. This provides two main advantages: How to do it

There are two APIs available in HTML5 that facilitate storage on the device.

Local Storage API
This API provides simple key = value persistent storage that fulfills many of the same needs as cookies but without the overhead of this information being transferred to the server on each request. It is appropriate for storing relatively simple, unstructured data.

The SQL Database API provides the ability to store and retrieve data in a local database . This is appropriate for more extensive and structured content.

[ HTML5 icon ] Requires: HTML5 Local Storage API and HTML5 Database Storage.

3.1.3 Replicate Local State Changes to a Server When Possible What it means

Local data storage is a powerful provision that allows Web applications to maintain a local data model. However, changes to the local data model are only visible on the device that made them and should be replicated to the server in order to provide a consistent view across devices.

See 3.5.10 Ensure Consistency Between Desktop and Mobile for more details. How to do it

HTML5 provides a property on the navigator object (navigator.onLine) to indicate whether the client is currently online, and dispatches two events on the Window object to indicate a change of network state (online and offline).

Mobile Web applications should use these events to control the asynchronous replication of data back to the server.

[ HTML5 icon ] Requires: HTML5 Local Storage API and HTML5 Database Storage.

3.2 Security and privacy

Use trusted information, and protect all personally identifiable information.

3.2.1 Do not Execute Untrusted JavaScript What it means

JSON is a very commonly used method to transfer data to the client, usually as part of an asynchronous request. However, using eval(); to directly execute the incoming datafeed represents a security risk and should be avoided.

Inadvertantly executing malicious JavaScript is particularly dangerous on mobile devices where personal information (current location, contact data, etc) may be exposed. How to do it

Instead of parsing JSON data by executing it with the browser's eval(); command, use a JSON parser instead (for example: http://www.json.org/json_parse.js).

In cases where directly evaluating the JSON data is desirable for reasons of efficiency, ensure that the data either contains no user-generated content (e.g. the server is responsible for all fields in the datafeed) or that any user-generated content is correctly escaped.

3.3 User Awareness and Control

Ensure that the user is aware of otherwise invisible application actions, and offer options to control those actions.

Browsers may have access to information such as:

Many browsers support the ability to make background server requests (i.e. requests that do not require action by the user). Unless specific action is taken to provide information about such activity, users may be exposed to revealing information that they did not intend to reveal and may be subject to unexpected data charges.

3.3.1 Inform the User About Automatic Network Access What it means

Whenever an application makes asynchronous XHR data requests, whether automatic (on a timer or in response to an external trigger) or secondary to some user action, this should be indicated to the user in an appropriate manner. How to do it

Applications should disclose how they use network resources. A simple icon indicating background activity is usually sufficient and does not interrupt the flow of the application. If extensive background network activity is required the user should be informed when they first visit the site, when they first sign-in, or in associated help pages.

The kinds of detailed information that could be disclosed in associated help pages or terms of service are:

3.3.2 Provide Sufficient Means to Control Automatic Network Access What it means

If an application makes automatic network requests (e.g. to poll the server for updates or to automatically store an updated client state) a means to control this activity must be provided. How to do it

All applications that access the network automatically must provide a means for the user to disable that activity. When automatic network activity is disabled, periodically prompt the user to make network requests.

Consider allowing the user to adjust the polling schedule and to control which activities are allowed to initiate network requests.

3.3.3 Ensure the User is Informed About Use of Personal and Device Information What it means

Ensure that the user is informed if the application needs to access personal or device information. The user should be informed of the types of information that will be used by the application and how that data will be exchanged with the server. How to do it

In many cases use of APIs that provide access to personal or device information causes a confirmation dialog to be presented to the user. In this case the application should not force the user to confirm again at the application level, but should make clear in the UI that displayed data has been accessed from the device. Need example of how to do this? and how should it know that a confirmation dialog is or is not presented?

If the user declines a prompt to allow application access to personal or device information, the application must recover gracefully. For example, if a request to a device API fails, do not automatically retry if this will lead to the user being presented with repeated native confirmation dialogues.

[ PIM DATA icon ] Requires: PIM Data APIs.

3.4 Conservative use of resources

Resources, such as device memory, processor power, and network bandwidth are significantly more limited on mobile devices than on the desktop. The most effective way to ensure that applications run smoothly and with low latency is to take steps to ensure minimal use of resources.

3.4.1 Use Transfer Compression What it means

Compress content for efficient delivery. How to do it

HTTP 1.1 compression, which uses the gzip and DEFLATE algorithms is widely supported. Web servers should be configured to serve appropriately compressed responses.

Note however, that the cost (in time and battery usage) of decompressing data should be balanced against the gains in transport efficiency. When configuring HTTP 1.1 compression note that:

3.4.2 Minimize Application Size What it means

This section elaborates on the Best Practices of BP1 (MINIMIZE). Smaller applications will download and execute more quickly and more reliably than larger ones on constrained devices. How to do it

Process HTML, JavaScript and CSS files to remove whitespace and minify before delivery. A number of open source whitespace strippers and JavaScript compilers are available.

The working group is seeking recommendations for the best / most-commonly used tools to reference in this section.

3.4.3 Avoid Redirects What it means

Request redirection (through HTTP response header or meta refresh) is typically used to exchange information between servers (e.g. account authentication). The delay incurred by redirects is much higher over mobile networks and so the number of redirects should be kept to a minimum to avoid degrading the user experience. How to do it

Try not to use more than two redirects for any request. If further redirects are required, use an interstitial page to communicate to the user that the application is still working.

3.4.4 Optimize Network Requests What it means

Network operations are costly in terms of battery usage, application latency, and potential network traffic expenses, and should not be made unnecessarily. The latency cost of setting up a HTTP request is much higher than the bandwidth limitations on a mobile network and so fewer, larger requests are preferred. How to do it

Consider the following possibilities when designing an application:

Batching requests:
A single request for more data is considerably more efficient than several smaller requests. Wherever possible, batch up multiple requests at the application level.
Back off during periods of inactivity:
If the application polls for updates, it should monitor user activity and poll less frequently during inactive periods.
Device Context:
If supported by the device, use awareness of current connectivity (e.g. WiFi) to select an appropriate level of interaction.

3.4.5 Minimize External Resources What it means

A Web application typically requires a number of resources (style sheets, scripts, image, etc) each of which may incur an additional HTTP Request. HTTP round trips are particularly expensive on a mobile network and so fewer, larger requests should be favoured over a larger number of smaller requests. How to do it

As far as makes sense after taking into account 3.5.8 Separate Rarely Used Functionality combine all style sheets into a single resource and all scripts into a single resource. If multiple scripts and style sheets are required as part of the authoring process, then try to arrange that they are merged before the page is served.

3.4.6 Consider Including Small Style Sheets and Script Resources In-Line What it means

In some circumstances performance is improved if JavaScript and CSS style sheet resources are included in the HTML page, eliminating the need for additional HTTP requests to fetch the external files. How to do it

Whether in-line resources make sense or not depends on a number of factors. For example:

3.4.7 Aggregate Static Images ("Sprites") into a Single Composite Resource What it means

Web applications often depend on a number of static images to provide icons, buttons, etc. If served as a separate image each one incurs an additional HTTP round-trip which is detrimental to performance when compared with combining them into a single image for transfer. How to do it

To optimize efficiency:

To render individual components of a resource use CSS positioning and clipping.

[ ICON: CSS ] Requires: CSS2 Clipping and Positioning Support

3.4.8 Include Background Images Inline in CSS Style Sheets What it means

Background images are often used as gradients to improve the look and feel of an application. These can be included in the CSS as base64 encoded strings in order to avoid an additional HTTP round trip.

Note that base64 encoding adds around 10% to the image size after gzip compression and this additional cost should be weighed against the benefits of less requests. How to do it

Background images can be encoded using the data URI scheme: url('data:image/png;base64, [data])

[ ICON: CSS ] Requires: RCF2397 data uri support.

3.4.9 Use Fingerprinting to Cache Dynamic Resources What it means

Dynamic resources that change occasionally (e.g. a user's avatar) can still be cached by identifying them with a URI that includes a fingerprint of the resource content. Using this technique means that the browser does not need to check the resource headers in order to validate its cache, instead, any change in the resource will lead naturally to a corresponding change in the resource reference. How to do it

3.4.10 Cache AJAX Data What it means

Data designed to be accessed by AJAX requests from the client should be cached in the same way as primary content. How to do it

The standard caching techniques (Expires header and Cache-Control header), as well as resource fingerprinting can be used on AJAX data as readily as primary content pages.

3.4.11 Use Power-Efficient Methods What it means

A Web application may not be suspended if it is placed in background. Consider the impact of on-going JavaScript operations on battery life. How to do it

The working group is currently investigating the relative impacts of Web application activities on battery life in order to make more concrete recommendations.

[ AC: I propose we cut this BP unless we can say something more concrete or empirically supported. I don't believe this BP is useful in its current form. ]

3.4.12 Minimize DOM Manipulation What it means

On small devices with limited processing capability, the cost of excessive DOM manipulation can impact application performance perhaps we can be either more specific (what does excessive DOM manipulation mean?) or considerably more terse here (e.g. DOM manipulation is costly). How to do it

Only use DOM manipulation for dynamic parts of the page. The static framework of the page is best created using HTML markup which can then be connected to the script using the element ID attribute. I'm not sure I quite get the point. Static parts of the page don't get manipulated and hence are not costly. Dynamic parts do get manipulated and hence have a cost. So what are we encouraging people to do, and what are we encouraging them not to do.

[ AC: I agree this BP is currently too vague to be useful and would prefer to cut it or come up with something more concrete to say ]

3.4.13 Reduce Cookie Size What it means

Information stored in cookies is sent to the server for every request and can negatively impact performance. How to do it

Use cookies sparingly, and consider alternative methods (see 3.1 Application Data).

3.4.14 Don't Send Cookie Information Unnecessarily What it means

Static resources don't need cookie information and so performance can be improved by serving these from a path or sub-domain for which the application's cookies are out of scope. How to do it

Use a different domain, sub-domain, or path name for static resources to the main application, and restrict the valid path of cookies such that they will not be exchanged when they are not needed.

For example:

Set-Cookie: somePreferenceInformation=purple; path=/myapp/

Application data served from /myapp will receive cookie information.

Static data served from /static will not receive unneeded cookie information.

3.5 User Experience

Given the additional complexities of interacting with an application on a mobile device, special consideration should be given to the overall user experience. User experience is influenced by a number of factors, including: perceived latency, interaction method, and data consistency.

3.5.1 Design for Multiple Interaction Methods What it means

Interaction methods vary across devices. Three main interaction methods should be considered when designing the UI:

The optimum configuration of UI elements varies depending on the interaction method used by the device. Ideally, the UI should be adapted based on a knowledge of the interaction methods supported by the target device. If this is not possible, then the UI should be carefully designed in order to provide a good experience in each of these different interaction methods. How to do it

Particularly where navigation of content requires multiple links (ie back/forward in a carousel) the following factors should be considered:

Focus Based:

Pointer Based:

Touch Based:

3.5.2 Use Scripting to Improve Perceived Performance What it means

Using script for dynamic parts of the page means that the view can be updated without a full page reload. Since reloading the entire page can be slow this can greatly improve application usability. How to do it

Use asynchronous (XHR) requests to get additional information from the server in response to user events and update the page DOM to convey this to the user.

AC: My feeling is that this BP is assumed by much of the other BPs, is too obvious, and doesn't provide value. Cut of change to: Avoid Page Reloads

3.5.3 Preserve Focus on Dynamic Page Updates What it means

The JavaScript focus method can be used to move the focus to the part of a page that has changed. However, if unexpected, this can confuse or irritate the user, especially if returning to the previous focus is not easy. How to do it

Use the JavaScript focus method only if it is essential to the use of the application, and does not inhibit user control/interaction.

3.5.4 Group Closely Coupled Views What it means

Most applications consist of a number of views (e.g. an email application consists of an inbox and the message detail view). User experience is improved if switching between these views does not require a request to the server. How to do it

Each view can be rendered in a div element which is hidden using CSS. JavaScript can be used to reveal content in response to user events.

AC: This feels like the wrong level of detail. Propose merging with 3.5.2 into Avoid Page Reloads

3.5.5 Use Fragment IDs to Maintain Application History What it means

Web applications can switch between views without a full page reload by showing and hiding sections of content. However, this means that the browser <back> button doesn't work by default, and it is not automatically possible to link directly to specific views within an application. Usability is enhanced by enabling both of these features: How to do it

Each view within an application should have a URI with a distinguishing fragment identifier (e.g. http://myapp.example.org/myapp#view) and JavaScript used to interrogate the browser location in order to determine which view to display.

3.5.6 Make Telephone Numbers "Click-to-Call" What it means

Standardized URI schemes have been defined for some common device functions, e.g. making phone calls and managing address books. These URI schemes, if supported, can enable users to easily use these functions from Web applications. How to do it

The most broadly supported scheme is tel: as described in [RFC3966]. Code such as the following can be used to enable "Click-to-Call":

<a href="tel:[PHONE-NUMBER]">[PHONE-NUMBER]</a>

3.5.7 Ensure Paragraph Text Flows What it means

On small screens it is important that paragraph text flows so that it doesn't require horizontal scrolling and so that it will reflow if the view orientation is changed. See BP1 [MEASURES] for more details. How to do it

Use percentage and measures for containers so that text can reflow automatically.

3.5.8 Separate Rarely Used Functionality What it means

Perceived performance can be improved by separating out JavaScript functions and CSS styles that are not required by the initial page. JavaScript functions and CSS styles associated with rarely used features should be bundled separately and downloaded only if those features are accessed. How to do it

Consider what functionality is being downloaded in a given script resource and how likely that functionality is to be used in the majority of requests. If some functionality is rarely used, it may make sense to partition this into a separate script to be loaded on demand. On-demand loading can be accomplished by:

The working group is seeking feedback on this Best Practice. Whilst it contains value for very large / complex applications, the implementation of this technique and its support across even high-end devices is not clear. Specifically, IFRAMEs are known to have significant issues on many devices and so we are looking for validation that their use in this context is a good idea.

3.5.9 Enable Progressive Rendering What it means

Progressive rendering means that the page will be displayed incrementally as it loads. In most cases this results in an improved perceived latency since content is available earlier. How to do it

Place CSS style sheet (<style>) elements in the <head> head element at the top of the document according to HMTL style elements cannot occur anywhere else :-) and what do we want to say about link rel=stylesheet? and JavaScript (<script>) elements at the bottom of the document. Browsers will not progressively render a page until the stylesheet has been loaded and will block while JavaScript content is parsed.

3.5.10 Ensure Consistency Between Desktop and Mobile What it means

This recommendation builds on the recommendation in BP1 (5.5.1 Thematic Consistency) and expands it to consider the application preferences and personalization data that form part of the overall experience on a mobile Web application.

User credentials valid on one device should be valid on other devices. User preferences captured on one device should be accessible on other devices. The most valuable example of this would be in offering a consistent experience where information entered during a desktop session is accessible in a mobile session and vice-versa. How to do it

For application data to be shared between devices it must be stored on a server and cannot be stored locally on a device (e.g. using cookies or a local data store). For any application data that is not exclusively relevant to the current device, favor storing it on the server so it can be shared by other devices. See 3.1 Application Data for more details.

3.5.11 Consider Using Push Technology For Notifications What it means

Network-initiated content delivery ("push") methods allow notifications and updates to be pushed to user even when they are outside of the application context. How to do it

Push method support may be disclosed through a User Agent Profile document if published by the device vendor, or through other device classification repositories.

If supported by the user agent, options for Push methods include:

3.5.12 Use Canvas Tag For Dynamic Graphics

TODO: Pending Jeff's research :)

3.5.13 Use viewport Meta Tag To Identify Desired Screen Size What it means

Certain classes of browser attempt to display desktop pages on a small screen by automatically zooming the display. This can be problematic for applications that have already been optimized for a small screen. The viewport meta tag tells the device at what scale to render the page. How to do it

A typical viewport setting looks like this:

<meta name="viewport" content="width=320; initial-scale=1.0; maximum-scale=1.0; user-scalable=0" /> ,

and should be inserted into the <head> of the HTML document. This setting informs the browser to always render the page at 100% (e.g. no browser based scaling) and explicitly disallows scaling of the page. Explicitly disallowing scaling is required to prevent the page being scaled when an input box is clicked on.

The setting above is appropriate for pages specifically designed for the target screen-size.

3.6 Handling Device Capability Variation

Device capability variation is a basic characteristic of the mobile Web environment. Web applications should adapt their content such that they render as well as possible on as broad a range of target devices as possible.

3.6.1 Use Server-side Capability Detection for Static Device Capabilities What it means

For static device capabilities that won't change (e.g. SVG support, screen-dimensions) it is preferable to detect these capabilities on the server and adapt content before it is sent to the client in order to avoid transferring unnecessary data. How to do it

Typically used methods of device capabilities detection:

3.6.2 Use Client-side Capability Detection for Dynamic Device State What it means

For dynamic device state that might depend on the configuration or context of the device (e.g. Is scripting enabled? Is the SDCard available? Has permission been granted for PIM access?) detection must be done on the device. How to do it

Use JavaScript reflection to determine if a given API is active and interrogate the device configuration using appropriate APIs. Two methods can then be used to adapt on the client to differing configurations:

  1. Encapsulate the different behaviours in the control logic of the application. E.g. simply use: if (some_configuration_variable) decision-points in the code and behave accordingly;
  2. Use an initial "bootstrap" script to assess device capabilities and request the appropriate application bundle accordingly.

Option (1) is simpler to implement and is appropriate provided the amount of inactive code downloaded doesn't have a negative impact on performance. Option (2) is preferred when the application must change significantly in response to properties that can only be determined on the client.

3.6.3 Use Device Classification to Simplify Content Adaptation What it means

If a large number of devices are being targeted, or the application is sensitive to the permutations of a large number of configuration properties, the number of application variants required might quickly become unmanageable.

To combat this, classify target devices into different device classes and build a single application variant for each.

This will keep the amount of device-specific code to a minimum without unduly encouraging a "lowest common denominator" solution. How to do it

Identify the target devices for the application and assign these to device classes of varying capability. Focus on application variants that work in each class rather than building device-specific exceptions for every variation in device configuration.

Device classes should be defined on an application-specific basis, so that the variants can be tailored accordingly. For example, the following is a typical configuration of application classes:

Class 1: Basic XHTML support, no or very basic scripting. No AJAX support.

Class 2: Full AJAX and JavaScript support.

Class 3: Advanced device APIs, for example: access to location API, device PIM data, or application cache.

3.6.4 Support a non-JavaScript Variant if Possible What it means

Scripted and XHR based applications are not yet well supported on many browsers. If possible, provide a variant of the application that does not rely on script by using synchronous FORM posts instead. This Best Pratice is related (albeit with a differing focus) to BP 1 [ OBJECTS_OR_SCRIPT]. How to do it

Essentially this BP states that it is favourable to support "Class 1" devices as defined above if at all possible. Doing this will ensure that the application can be used across as broad a range of devices as possible. Furthermore, in some cases a non-JavaScript version can sometimes be useful for simple operations in low-bandwidth situations.

In some cases, however, the type of application simply has no non-JavaScript counterpart (e.g. a Web based game, an Instant Messaging client) in which case it should return a 406 Not Acceptable response.

Do this by detecting the device User-Agent and checking its JavaScript support against a DDR or local index.

3.6.5 Offer Users a Choice of Interfaces What it means

Not only is device characteristic detection imperfect, it cannot always account for the differing use-cases of an application. If multiple flavours of the application exist (e.g. to support the various device classifications) it sometimes makes sense to offer the user the choice of which flavour they wish to use. How to do it

Only if it makes sense in the specific context of a given application, allow the user to switch to a different flavour (for example, upgrading their experience if their device is more capable than the server believes, or degrading if connectivity is poor and they wish to accomplish a very simple task that can be done more easily with the minimal UI).

Always attempt to default to the most appropriate UI on first use.

Always remember the user's preference for future visits in a cookie or local data store.

Appendix: Best Practice Dependent Device Properties

The following device properties included in the DDR Core Vocabulary [REF] are of particular value in supporting best practices recommended in this document. They should be available in any DDR supporting the W3C's DDR Core Vocabulary:

Appendix: Examples

TODO: Include more detailed examples on the following BPs... Use Fragment IDs

A Sources (Non-Normative)

The Best Practice statements have been assembled by the BPWG from a number of sources. Primary among those are:

While the Best Practice statements have mainly been assembled by secondary research, the sources for that research have in many cases been assembled from primary research. In addition, group members' contributions are to some extent informed by primary research carried out by their company.

B Related Reading (Non-Normative)

Readers interested in the topic of this document will find a variety of other publications of interest. As noted in the Scope paragraph above, topics such as internationalization and accessibility have been addressed separately by the W3C and have not been covered here.

The Character Model for the World Wide Web and other materials prepared by the W3C Internationalization (i18n) Activity cover important interoperability drivers for content prepared for the One Web and the mobile-services arena.

The Web Accessibility Initiative has prepared a variety of Guidelines and Techniques that likewise bear on the preparation and processing of content in and for the Web.

Section 3.6.3 Use Device Classification to Simplify Content Adaptation above introduced the idea of content adaptation. Readers who contemplate implementing server-side adaptation will be interested in the ongoing work of the Device Independence Activity.

C Acknowledgements (Non-Normative)

D References (Non-Normative)

D.1 MWI References

to be added

D.2 Sources

to be added

D.3 Device Independence

Device Independence Principles, R. Gimson, Editor, W3C Working Group Note, 1 September 2003 (See http://www.w3.org/TR/2003/NOTE-di-princ-20030901/)
Delivery Context Overview for Device Independence, , R. Gimson, R. Lewis, S. Sathish, Editors, W3C Working Group Note, 20 March 2006 (See http://www.w3.org/TR/di-dco/)
Glossary of Terms for Device Independence, R. Lewis, Editor, W3C Working Draft (work in progress), 18 January 2005 (See http://www.w3.org/TR/2005/WD-di-gloss-20050118/)

D.4 Web, Protocols and Languages

Extensible Markup Language (XML) 1.0 (Fifth Edition), Tim Bray, Jean Paoli, C. M. Sperberg-McQueen, Eve Maler, Fran