Copyright © 2009 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.
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 (BP1) [MWBP], especially those 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.
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 is a Last Call Working Draft of Mobile Web Application Best Practices, expected to become a W3C Recommendation. The W3C Membership and other interested parties are invited to review the document and send comments to public-bpwg-comments@w3.org (with public archive) through 6 November 2009.
This document was developed by the Mobile Web Best Practices Working Group as part of the Mobile Web Initiative. A complete list of changes since publication as a Working Draft on 7 May 2009 is available. Main changes are:
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 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.
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 Sparingly
3.1.2 Use Appropriate Client-Side Storage Technologies
for Local Data
3.1.3 Replicate Local Data
3.2 Security and privacy
3.2.1 Do not Execute Unescaped or Untrusted JSON
data
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
3.3.4 Enable Automatic Sign-in
3.4 Conservative use of resources
3.4.1 Use Transfer Compression
3.4.2 Minimize Application and Data Size
3.4.3 Avoid Redirects
3.4.4 Optimize Network Requests
3.4.5 Minimize External Resources
3.4.6 Aggregate Static Images into a Single
Composite Resource (Sprites)
3.4.7 Include Background Images Inline in CSS Style
Sheets
3.4.8 Cache Resources By Fingerprinting Resource
References
3.4.9 Cache AJAX Data
3.4.10 Don't Send Cookie Information Unnecessarily
3.4.11 Keep DOM Size Reasonable
3.5 User Experience
3.5.1 Optimize For Application Start-up
Time
3.5.2 Minimize Perceived Latency
3.5.3 Design for Multiple Interaction Methods
3.5.4 Preserve Focus on Dynamic Page
Updates
3.5.5 Use Fragment IDs to Drive Application View
3.5.6 Make Telephone Numbers
"Click-to-Call"
3.5.7 Ensure Paragraph Text Flows
3.5.8 Ensure Consistency Of State Between Devices
3.5.9 Consider Mobile Specific Technologies for
Initiating Web Applications
3.5.10 Consider Use Of Canvas Tag or SVG For Dynamic
Graphics
3.5.11 Use viewport Meta Tag To Identify Desired Screen
Size
3.6 Handling Variations in the Delivery Context
3.6.1 Prefer Server-Side Detection Where
Possible
3.6.2 Use Client-Side Detection When
Necessary
3.6.3 Use Device Classification to Simplify
Content Adaptation
3.6.4 Support a non-JavaScript Variant
if Appropriate
3.6.5 Offer Users a Choice of Interfaces
Appendix: Best Practice
Dependent Device Properties
A Related Reading (Non-Normative)
B References (Non-Normative)
B.1 MWI References
B.2 Device Independence
B.3 Web, Protocols and Languages
B.4 Other References
The following Best Practices are discussed in this document and listed here for convenience.
Use Appropriate Client-Side Storage Technologies for Local Data
Provide Sufficient Means to Control Automatic Network Access
Ensure the User is Informed About Use of Personal and Device Information
Aggregate Static Images into a Single Composite Resource (Sprites)
Consider Mobile Specific Technologies for Initiating Web Applications
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.
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 (BP1) [MWBP].
The document is not targeted solely at developers; others, such as interaction and graphic designers, site administrators, and tool developers are encouraged to read it.
These recommendations expand on the recommendations of BP1. 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.
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).
In building a Web application, it's not necessary to implement all Best Practices in order to avoid pathological behaviour. Instead, each Best Practice should be considered as a possible measure that might be implemented towards the goal of providing as rich and dynamic an experience as possible on a mobile Web browser.
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 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 locally executable 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 as part of the Web Widgets Effort [WIDGETS] and also in a number of vendor-specific initiatives.
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, intermittent connectivity), and also the additional scope and features that should be considered when developing for the mobile context (e.g. device context / location, presence of personal data on the device, etc).
Requirements on delivery context have not been made explicitly, but most Best Practices assume devices with support for standard XHTML, JavaScript, and CSS capability. At the time of writing, developers of relatively complex Web applications targetting mid- to high-end devices are most likely to benefit from these Best Practices, but as the technology evolves it is expected that the range of relevant devices will increase.
Additionally, some Best Practices are relevant only if the device exposes certain capabilities (for example, access to device information such as location). In these cases the requirements are summarized as a separate sub-heading.
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 Handling Variations in Delivery Context.
These recommendations are complementary to the recommendations of Mobile Web Best Practices 1.0 (BP1) though their focus is somewhat orthogonal. Whereas BP1 focussed on delivering a good experience on a broad range of devices, this document's focus is on making use of advanced device capabilities to deliver the best possible experience on those devices that can support it. For this reason, whilst readers of this document are likely to benefit from reading BP1, BP1 is in no way a pre-requisite for this document.
This document builds on some of the concepts described by the Ubiquitous Web Applications Working Group (UWA) and the Device Independence Principles [DIP]. It also 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].
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.
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.
Cookies are a common and effective means to store small amounts of state on the client. They are appropriate for simple personalization data and are commonly used to store a token representing user identity in order to enable automatic sign-in.
Information stored in cookies, however, is sent to the server for every request and so using them for excessive amounts of data can negatively impact performance, particularly on a mobile network.
Also, in the mobile context, cookie support cannot be relied upon since it may be disabled either in the device configuration or by the mobile network. For this reason, applications should endeavour to remain functional even if cookies are unavailable. See BP1 [COOKIES] Do not rely on cookies being available for more cookie related caveats.
If supported by the device, client-side storage APIs provide a mechanism to store more extensive amounts of data than would be appropriate with cookies. Some examples of technologies that support client-side storage APIs are BONDI [BONDI], HTML5 [HTML5], and Opera Widgets [OPERA].
Making use of client-side storage in Web applications is a powerful technique that brings Web applications into parity with native applications in terms of start-up time and responsiveness. Two key advantages are worth noting explicitly:
[ Client-Side Storage ] Requires: Local Storage API.
If a client-side storage API is being used the data in it is not visible to other devices. Whilst this is appropriate for some forms of data (e.g. preferences and state relevant only to a given device), it is often necessary to replicate this data back to a server in order to provide a consistent view across devices and make it possible to recover data if the device is lost or damaged. See 3.5.10 Ensure Consistency Of State Between Devices for further discussion on this topic.
As a rule of thumb, data that needs to be shared with other devices or recovered in the case of a lost or damaged device, should be replicated back to the server as soon as possible.
The technologies that provide client-side storage APIs provide facilities to detect the current network connectivity. For example, 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).
However, these APIs should be used with caution. Even if the browser is reporting an online state, on an intermittant network this is no guarentee that a subsequent connection will succeed. The most effective approach is to fail gracefully in the event of a connection failure, store unsaved data in a queue of uncommitted changes, and set a timer to try again later.
[ Client-Side Storage ] Requires: Local Storage API.
Use trusted information, and protect all personally identifiable information.
3.2.1.1 What it means
A common pattern is to use JSON to transfer data to a client and then use JavaScript's eval() function to parse it. This is a powerful technique, since on constrained devices eval() can execute more quickly than the alternatives. However, direct execution of a datafeed that contains unescaped user-generated data represents a significant 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.
3.2.1.2 How to do it
Where possible, instead of parsing JSON data by executing it with the eval() function, use a JSON parser (for example: http://www.json.org/json_parse.js).
If this is impractical (JSON parsers are approximately x10 slower than eval()and may be impractical for large datafeeds) ensure that the data contains no user-generated content (e.g. the server is responsible for the content of all fields in the datafeed) or that any user-generated content is correctly escaped.
See RFC4627 for details on how to ensure a JSON datafeed is suitably escaped and can be safely passed into JavaScript's eval() function.
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:
Network traffic on a mobile device incurs a cost (both in terms of data-charges and battery life) and so it is important to inform the user when this happens. 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 so that the user..
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:
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 should be provided.
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.
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 whether / how that data will be exchanged with the server.
These notices should be provided when the user first accesses the Web application, on first access to user information, or in help pages. It should provide the user with enough information to reasonably judge whether or not they want to allow the application access to their data.
In many cases use of APIs that provide access to personal or device information causes a native 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.
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.
[ DEVICE DATA ] Requires: Device Data APIs.
If an application requires user identity it is usual to prompt for user credentials (username and password) and provide the option to sign-in automatically on next usage session. This is especially important on a mobile device where data input is more painful than on a desktop.
User credentials can be stored in a cookie or in local storage. However, it is important not to store unencrypted password information since this is insecure. Most commonly, a securely hashed token which can be revoked on the server if necessary is stored locally in order to enable automatic sign-in.
Battery lifetime and cost of network traffic are significant considerations of most users of mobile devices. Since all activities that use either processor or wireless connectivity will incur some cost to battery life or network data costs, applications should consider this factor and be conservative in their use of resources.
Additionally, 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.
Compress content for efficient delivery.
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:
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.
Process HTML, JavaScript and CSS files to remove whitespace and minify before delivery. A number of freely available whitespace strippers and JavaScript / CSS optimizers are available online.
Note that "minification" / "optimization" can take a number of forms from simple removal of whitespace and comments, to the global substitution of tokens (variables, method names, selector names) with shorter alternatives.
In general, minification that parses the source file and makes substitutions based on a lexical / grammatical understanding of that source are less fragile and should be preferred to simple regular-expression based tools.
For a good comparison of JavaScript minification tools try: http://compressorrater.thruhere.net
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.
Try not to use redirects. If more than two redirects are required consider using an interstitial page to communicate to the user that the application is still working.
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.
Consider the following possibilities when designing an application:
3.4.5.1 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.
3.4.5.2 How to do it
As far as makes sense after taking into account 3.5.2 Minimize Perceived Latency 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.
3.4.6.1 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.
3.4.6.2 How to do it
To optimize efficiency:
To render individual components of a resource use CSS positioning and clipping.
[ CSS ] Requires: CSS2 Clipping and Positioning Support
3.4.7.1 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.
3.4.7.2 How to do it
Background images can be encoded using the data URI scheme:
url('data:image/png;base64, [data])
[ CSS ] Requires: RCF2397 data uri support.
3.4.8.1. 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 Hash 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.
3.4.8.2 How to do it
Expires header to a date in the far future.3.4.9.1 What it means
If possible, data designed to be accessed by AJAX requests from the client should be cached in the same way as primary content.
3.4.9.2 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.
3.4.10.1 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.
3.4.10.2 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.
The permitted size of the Document Object Model (DOM) is more constrained on mobile devices and can be exceeded by large / complex pages. Keep the DOM size below 10MB to avoid browser crashes.
Clip content and separate content onto separate pages to keep the DOM size manageable.
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: latency, interaction method, and data consistency.
User experience is strongly influenced by the initial start-up time of an application.
Offline Web application technologies like HTML5 AppCache bring Web applications into parity with native applications in terms of their start-up time and their ability to be used under intermittent network coverage. The following steps should be considered to minmize the start time of a Web application.
Consider the following techniques to help minimize application start time:
The Best Practices covered in section 3.4 Conservative use of resources will help to minimize the latency of a Web application, but a number of measures can also be used to further minimize the perceived latency. Lowering perceived latency is an important factor in improving the overall usability of a Web application, improving user's perception of Web site credibility and decreasing bail out rates.
A number of techniques can be used to improve perceived latency:
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.
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:
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.
Use the JavaScript focus method only if it is essential to the
use of the application, and does not inhibit user control/interaction.
3.5.5.1 What it means
Web applications can switch 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 possible to link directly to specific views within an application. Usability is enhanced by enabling both of these features:
3.5.5.2 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.
For further discussion on this topic see: http://ajaxpatterns.org/Unique_URLs
Note that showing and hiding content in this way can have adverse affects on accessibility if not carefully handled. See ARIA for more information on writing accessible rich Web applications.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.
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>
3.5.7.1 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.
3.5.7.2 How to do it
Use percentage and measures for containers so that text can reflow automatically.
3.5.8.1 What it means
This recommendation builds on the recommendation in BP1 (5.5.1 Thematic Consistency) and expands it to consider the application preferences, personalization data, and state 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. Data updated on one devices should be viewable consistently 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.
3.5.8.2 How to do it
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.
Network-initiated content delivery ("push") methods allow notifications and updates to be pushed to user even when they are outside of the application context.
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:
Canvas and SVG provide alternative options for incorporating graphics in a Web application. Support for these technologies varies across devices so in many cases the choice of which technology to use will depend on the target devices for a given application.
The Canvas element defines a drawable bitmap region onto which JavaScript can be used to render simple graphic primatives. In contrast, SVG is an XML language for defining vector graphics -- the nodes and elements are added to a DOM and can be modified later using JavaScript.SVG is well-suited for graphics that must be scalable and whose components need to be modified (e.g. panning and zooming a map) whereas Canvas is best suited for cases where a static bitmap is sufficient (e.g. drawing a scatter-chart, visual effects, reflections etc).
In most cases Canvas is faster and should be preferred if it meets requirements. However, since Canvas generates a flat bitmap it is not inherently accessible and so should not be used as the sole means of conveying information.
See http://dev.w3.org/html5/spec/Overview.html#the-canvas-element for information on how to use the Canvas tag.
See http://www.w3.org/Graphics/SVG/ for information on how to use SVG.
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.
A typical viewport setting looks like this:
<meta name="viewport" content="width=device-width,minimum-scale=1.0,maximum-scale=1.0"/> ,
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.
The setting above is appropriate for pages specifically designed for the target screen-size.
Variations in the delivery context (such as different device capabilities) is a prominent feature of the mobile Web. Web applications should adapt to known or discoverable properties of the delivery context by adjusting the content, navigation or page flow, with a view to offering a good user experience on as broad a range of devices as possible.
Where possible, use the evidence available on the server to determine the properties of the delivery context, and adapt the responses to the client before transmission, thus improving the user experience and avoiding transmission of unnecessary or incompatible data.
In its most basic form, the minimum evidence from the client device is the HTTP request header. Typically, the following header fields provide evidence of device capabilities:
Where it is not possible to determine certain properties of the delivery context from the server, this information may be available at the client. Once obtained at the client, the information can be used directly to adapt the presentation, or it can used to request alternative, adapted content from the server.
There are a few client-side solutions available to the developer:
JavaScript: this is the most common solution. A script determines the device / browser properties and manipulates the content and behaviour of the application accordingly. This can be done in two ways:
CSS Media Types: CSS Media Types allow different stylesheets to be associated with different media types (e.g. print, screen, mobile) and are traditionally used to repurpose content for these destinations. Since the list of recognized media types is limited, however, and devices are notoriously idiosyncratic in their interpretation of types, it is in general not a helpful technology in this context. See Media Types [CSSMT] for more details.
CSS Media Queries: Media queries are an extension to the "media-types" paradigm that allow developers to apply specific style rules based on the device display characteristics (e.g. screen width, orientation, or resolution). At the time of writing this specification is not fully supported, but can provide a useful way to modify the page layout (for example to reflow sections of text) in a more maintainable, declarative way than is possible with script. See Media Queries [CSSMQ] for more details.
If a large number of devices are being targeted, or if 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.
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 possible configuration of application classes:
Class 1: Basic XHTML support, no or very basic scripting. No XHR support. (Even if these kind of devices are not being explicitly supported, it is often advisable to support a non-XHR version in case JavaScript has been disabled on the device).
Class 2: Full AJAX and JavaScript support.
Class 3: Advanced device APIs, for example: access to location API, device PIM data, or application cache.
Scripted and XHR based applications are not supported on all browsers. If broadest reach is a primary concern then consider providing a variant of the application that uses synchronous FORM posts in place of XHR requests. This Best Pratice is related (albeit with a differing focus) to BP 1 [ OBJECTS_OR_SCRIPT].
Essentially this BP states that it is favourable to support "Class 1" devices as defined above if appropriate. 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 be useful for simple operations in low-bandwidth situations.
In some cases, however, a particular 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.
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.
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.
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:
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.
Mobile Web Best Practices 1.0, Jo Rabin, Editor, W3C Recommendation, 29th July 2008 (see http://www.w3.org/TR/mobile-bp/)