Mobile Web Application Best Practices

W3C Proposed Recommendation 21 October 14 December 2010

This version:

http://www.w3.org/TR/2010/PR-mwabp-20101021/ http://www.w3.org/TR/2010/REC-mwabp-20101214/

Latest version:

http://www.w3.org/TR/mwabp/

Previous version:

http://www.w3.org/TR/2010/WD-mwabp-20100713/ http://www.w3.org/TR/2010/PR-mwabp-20101021/

Editors:

Adam Connors, Google

Bryan Sullivan, AT&T (until 2008)

Please refer to the errata for this document, which may include some normative corrections.

See also translations .

Copyright ©2010 W3C ^® ( MIT , ERCIM , Keio ), All Rights Reserved. W3C liability , trademark and document use rules apply.

3.1.3 Replicate Local Data To A Server If Necessary

3.1.3.1 What it means

If a client-side storage API is being used the data in it is not visible to the user's 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 send this data back to a server in order to provide a consistent view across devices (e.g. between mobile and desktop instances of an application) and make it possible to recover data if the device is lost or damaged. See 3.5.8 Ensure Consistency Of State Between Devices for further discussion on these topics.

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.

3.1.3.2 How to do it

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 intermittent network this is no guarantee 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.

3.2 Security and privacy

Use trusted information, and protect all personally identifiable information. Mobile Web applications are subject to the same security considerations and risks as desktop Web applications, and as such most desktop related security advice is applicable to mobile. (For example, see OWASP [ OWASP ] for a good summary of common Web security Best Practices). This document does not attempt to provide an exhaustive survey of security issues but the following Best Practice has been called out on the grounds that it is specifically relevant to mobile.

3.2.1 Do not Execute Unescaped or Untrusted JSON data

3.2.1.1 What it means

A common technique 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.

Inadvertently 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 a JavaScript implementation of a JSON parser [ JSON-PARSE ]).

If this is impractical 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 [ RFC4627 ] for details on how to ensure a JSON datafeed is suitably escaped and can be safely passed into JavaScript's eval() function.

3.3 User Awareness and Control

Allow the user to control application behaviour that might not otherwise be apparent, such as access to the network and access to device data. For example:

• Pictures, music, and video clips;
• Contacts, calendar (PIM data);
• Call history;
• System data (battery, coverage, roaming, location);
• Media recording (record audio/video clip, get new picture);
• Device context (e.g. location, connectivity, profile setting).

Note that where possible it is preferable to rely on the browser's native functionality to notify the user of these activities, however the Best Practices below provide further advice on appropriate application behaviour in situations where the native functionality of the browser may not be sufficient.

3.3.1 Ensure the User is Informed About Use of Personal and Device Information

3.3.1.1 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 whether / how that data will be exchanged with the server.

These notices should be provided when the user first accesses the Web application, or on first access to user information. It should provide the user with enough information to judge whether or not they want to allow the application access to their data.

3.3.1.2 How to do it

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 dialog boxes.

[ DEVICE DATA ] Requires: Device Data APIs.

3.3.2 Enable Automatic Sign-in

3.3.2.1 What it means

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 difficult than on a desktop.

Note that if automatic sign-in is enabled, a sign-out link should also be provided.

3.3.2.2 How to do it

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. Typically, a securely hashed token which, if necessary, can be revoked on the server, is stored locally in order to enable automatic sign-in.

3.4 Conservative use of resources

The most effective way to ensure that applications run smoothly and with low latency is to minimize use of device memory, processor power, and network bandwidth which are more limited on mobile devices than on the desktop.

3.4.1 Use Transfer Compression

3.4.1.1 What it means

Compress content for efficient delivery.

3.4.1.2 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. Where supported, alternative compression formats (such as EXI [ EXI ]) may also provide benefit.

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:

• Most image formats (especially JPEGs) do not benefit from compression, but SVG does;
• Most other media formats (e.g. audio, video) do not benefit from compression;
• Very small files (e.g. <1k) generally do not benefit from compression.

Where supported, alternative compression formats (such as EXI [ EXI ]) that do not share some of the above impediments, may provide benefit.

3.4.2 Minimize Application and Data Size

3.4.2.1 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.

3.4.2.2 How to do it

Process HTML, JavaScript and CSS files to remove whitespace and minify before delivery.

"Minification" / "optimization" may take a number of forms from simple removal of white space and comments, to the global substitution of tokens (variables, method names, selector names) with shorter alternatives. In general, minification based upon a lexical / grammatical understanding of that source is less fragile and is preferred to simple regular-expression based tools.

See the JavaScript CompressorRater [ JSCOMP ] for a comparison of JavaScript minification tools.

3.4.4 Optimize Network Requests

3.4.4.1 What it means

Establishing the necessary connections in order to complete an HTTP request can take significantly longer on a mobile network than on a fixed network. Even though bandwidth is typically more restricted on a mobile network it is still preferable to make fewer, larger requests.

3.4.4.2 How to do it

Consider the following possibilities when designing an application:

Batching requests:

Since a single request for more data is likely to provide a better user experience than several smaller requests, wherever possible, batch up multiple requests at the application level.

Throttle low-priority requests:

In some applications certain requests may be less critical than others (e.g. logging requests). Throttle low-priority requests to ensure they don't block the network and prevent more critical requests from being serviced quickly.

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

3.4.5.1 What it means

A Web application typically requires a number of resources (style sheets, scripts, image, etc) each of which requires an HTTP request. As above, HTTP requests are particularly expensive on a mobile network and so fewer, larger requests should be favored 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 Aggregate Static Images into a Single Composite Resource (Sprites)

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 request which is detrimental to performance.

3.4.6.2 How to do it

Define candidate images as CSS background images and combine them into a single image for transfer (spriting). To optimize efficiency combine images of similar sizes and color palettes. Also, combine images that do not change often, since if one of the component images changes the entire combination image will need to be refreshed. To render individual components of a resource use CSS positioning and clipping.

Note that this technique should only be applied to decorative images (e.g. those that don't require an alt text). Spriting informational images leads to an undesirable coupling of content and layout and is detrimental to accessibility.

[ CSS ] Requires: CSS2 Clipping and Positioning Support

3.4.7 Include Background Images Inline in CSS Style Sheets

3.4.7.1 What it means

Visual effects (e.g. background images and gradients) are often used to improve the look and feel of an application. These can be included in CSS as base64 encoded strings in order to avoid an additional HTTP request

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 fewer 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: RFC2397 [ RFC2397 ] data uri support.

3.4.8 Cache Resources By Fingerprinting Resource References

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.

For example,

<img src="http://www.example.com/userimages/joeblogs-67f90da089da">

Where the actual resource at joeblogs-[xxx] could be either generated in some offline process or served dynamically.

3.4.8.2 How to do it

• Set the resource caching policy to "never expire" by setting the Expires header to a date in the far future.
• Reference the resource using a URI that contains a Hash of the content. If the content changes, this reference will change and the browser will fetch the updated data.

3.4.9 Cache AJAX Data

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 (see 3.4.8 ) can be used on AJAX data as readily as primary content pages.

3.4.10 Do not Send Cookie Information Unnecessarily

3.4.10.1 What it means

Static resources do not need cookie information and so performance can be improved by serving them 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:

Application data served from /myapp will receive cookie information.

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

3.4.11 Keep DOM Size Reasonable

3.4.11.1 What it means

The in-memory size of the Document Object Model (DOM) may be limited on mobile devices. Large / complex pages may exceed this limit and cause unpredictable errors.

3.4.11.2 How to do it

Limit the amount of information in the DOM at a single time using pagination or other appropriate techniques.

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: latency, interaction method, and data consistency.

3.5.1 Optimize For Application Start-up Time

3.5.1.1 What it means

User experience is strongly influenced by the initial start-up time of an application.

Offline Web application technologies like HTML5 AppCache [ HTML5-OFFLINE ] bring Web applications into parity with native applications in terms of their start-up time and their ability to be used even where network coverage is intermittent. The following steps should be considered to minimize the start time of a Web application.

3.5.1.2 How to do it

Consider the following techniques to help minimize application start time:

• Use Offline Technology: Offline Web technologies (for example, AppCache) allow the resources of a Web application (its HTML, JavaScript, and CSS files) to be specified and stored locally so that the application can start without requiring a round-trip to the server.
• Consider Partitioning Large Scripts: In complex Web applications, JavaScript parsing can contribute a significant portion of start time. If some functionality is rarely used it should be moved into separate scripts that can be loaded on demand, lowering the amount of core code that needs to be parsed at start-up.
• Use Local Storage: Where appropriate, store a snapshot of the last application state so it can be displayed immediately on start-up without requiring a server roundtrip.
• Minimize Number of Local Storage Queries: The number of local storage queries required to generate the initial view is a significant contribution to start-up latency. Try to minimize the number of local storage queries required before the first view can be displayed.

3.5.2 Minimize Perceived Latency

3.5.2.1 What it means

Lowering perceived latency is an important factor in improving the overall usability of a Web application.

3.5.2.2 How to do it

A number of techniques can be used to lower perceived latency:

• Enable Incremental Rendering : Place JavaScript at the bottom of the page (since browsers rendering halts while parsing JavaScript) and configure the page so that any useful information that might be available is viewable while the main content of the application is still loading.
• Keep the User Informed of Activity: Use spinners progress bars to keep the user informed during network and device API accesses so that they do not think the application is halted.
• Avoid Page Reloads: To reflect changes in state or show different views within an application, update pages dynamically (by manipulating the DOM) rather than reloading them.
• Preload Probable Next Views: Preload data for frequently traversed paths in the application so it can be displayed more quickly when the user requests it.

3.5.3 Design for Multiple Interaction Methods

3.5.3.1 What it means

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

• Focus Based: The browser focus "jumps" from element to element;
• Pointer Based: Key-based navigation controls a pointer that can cover any part of the screen;
• Touch Based: Events are related directly to a finger or stylus touch position on the screen.

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 designed to provide a good experience for each of these different interaction methods.

Additionally note that (as always) new interaction methods are likely to emerge in the future, particularly in the fields of voice and assistive technology. For additional, non-mobile specific, guidelines on accessibility and designing for assistive technologies, see Web Content Accessibility Guidelines (WCAG) 2.0 [ WCAG20 ].

3.5.3.2 How to do it

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

Focus Based:

• The current focus of the page is easily determined because the focus element will be highlighted;
• Focus area will jump automatically from one selectable element to another (e.g. from link to link) without affecting usability even when widely spaced.

Pointer Based:

• Selectable elements that are associated with each other need to be close as moving the pointer can be slow;
• Selectable elements need to be large enough to be easily selected -- since the pointer often moves in steps of between 5 - 10 pixels;
• Selectable elements should have rollovers to make it clear when the pointer has entered their active area.

Touch Based:

• Selectable elements may be (but don't have to be) widely spaced since the user can select them directly;
• Selectable elements must be large enough to be easily selected (e.g. list items should have a screen height of at least around 1cm);
• No elements are in focus until they are selected so extra information cannot be passed to the user (e.g. rollovers will not work).

3.5.4 Preserve Focus on Dynamic Page Updates

3.5.4.1 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.

3.5.4.2 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.

3.5.5 Use Fragment IDs to Drive Application View

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:

• Enabling deep links (e.g. to the content of a specific email) means the user can bookmark this view and return to it quickly;
• Enabling the browser history provides a natural method to navigate application views that is natively supported by the browser.

3.5.5.2 How to do it

Assign a URI with a distinguishing fragment identifier (e.g. http://myapp.example.org/myapp#view ). Use JavaScript to interrogate the browser location in order to determine which view to display.

For further discussion on this topic see: Unique URLs pattern [ AJAX-UNIQUE ] on the Ajax Design Patterns Web site.

Note that showing and hiding content in this way can have adverse affects on accessibility if not carefully handled. See WAI-ARIA [ ARIA ] for more information on writing accessible rich Web applications.

3.5.6 Make Telephone Numbers "Click-to-Call"

3.5.6.1 What it means

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

3.5.6.2 How to do it

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

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

Note that [PHONE-NUMBER] should always be entered using the full international prefix (e.g. +1-201-555-0111) to ensure that it works outside of its home country.

Similarly RFC5724 [ RFC5724 ] can be used to send a GSM SMS (text message) as follows:

<a href="sms:[PHONE-NUMBER]?body=[MESSAGE]">[PHONE-NUMBER]</a>

Note that at the time of writing support for this RFC is limited and device compatibility should be verified before deployment.

3.5.7 Ensure Paragraph Text Flows

3.5.7.1 What it means

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

3.5.7.2 How to do it

Do not use absolute or pixel measures. Use percentage and RELATIVE measures for containers so that text can re-flow automatically.

3.5.8 Ensure Consistency Of State Between Devices

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 device should be viewable consistently on other devices.

An important example of this is offering a consistent experience where data entered on a desktop is available on a mobile 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.

3.6 Handling Variation in the Delivery Context

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.

3.6.1 Prefer Server-Side Detection Where Possible

3.6.1.1 What it means

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 transfer, thus improving the user experience and avoiding transfer of unnecessary or incompatible data.

3.6.1.2 How to do it

In its most basic form, the minimum evidence from the requesting device is the HTTP request header fields. Typically, the following header fields provide evidence of device capabilities:

• Accept: this list of MIME types can aid in the selection or creation of alternative content representations to suit the requesting device. This field is not always reliable however, and its value often includes "*/*", suggesting that clients can accept every MIME type.
• User-Agent: as a generally unique (albeit opaque) string it can be used as a key into a device description repository [ DDR ]. The set of properties recorded in these repositories varies from implementation to implementation. The W3C DDR Simple API defines a common interface and a means of expressing the vocabulary of properties for such repositories.
• X-Wap-Profile: this is a reference to the User Agent Profile [ UAPROF ] for the requesting device. In practice, the referenced profile is not always guaranteed to be available, valid or up-to-date, so the value of this field is sometimes used with a DDR where corrections to the profiles are stored. Some devices may send an additional field X-Wap-Profile-Diff advertising temporary or permanent variations of a specific device with respect to its standard profile.

3.6.2 Use Client-Side Capability Detection Where Necessary

3.6.2.1 What it means

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.

3.6.2.2 How to do it

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:

1. By encapsulating the differing behaviors in the control logic of the application (e.g. if (some_api_exists) { ... ). Typically the Delivery Context information is gathered at the start of the session, though dynamic information (e.g. current screen orientation) should be refreshed during the session.
2. By passing the gathered information back to the server and requesting alternative content (e.g. either by dynamically adding a new <script> element to the DOM or by an XHR request).

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 re-flow sections of text) in a more maintainable, declarative way than is possible with script. See Media Queries [ CSSMQ ] for more details.

3.6.3 Use Device Classification to Simplify Content Adaptation

3.6.3.1 What it means

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 and build a single application variant for each class. This allows you to exploit device capabilities with a manageable code base.

3.6.3.2 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 by application basis, so that the variants can be tailored accordingly.

Example 1: Possible definition of application classes based on rendering and device API capabilities:

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.

Example 2: Possible definition of application classes based on supported user-interaction modes:

Class 1: Pointer Based.

Class 2: Touch Based.

3.6.4 Support a non-JavaScript Variant if Appropriate

3.6.4.1 What it means

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 Practice is related (albeit with a differing focus) to BP 1 [ OBJECTS_OR_SCRIPT] .

3.6.4.2 How to do it

Essentially this BP states that it is favorable to support "Class 1" devices as defined in the first example 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 the server should return a response with human readable content explaining the situation in more detail.

Ideally, content should be adapted before transfer by checking whether or not the device supports JavaScript using a DDR or local index. However, in some cases (e.g. if the device has disabled JavaScript) JavaScript may still be sent to a device that can't process it. To cover this case, a <noscript> element should always be included and contain a suitably informative message.

3.6.5 Offer Users a Choice of Interfaces

3.6.5.1 What it means

Not only is device characteristic detection imperfect, it cannot always account for the differing use cases of an application. Therefore automatic detection is not sufficient as the only mechanism for determining which version is appropriate.

3.6.5.2 How to do it

Where multiple versions of an application exist (e.g. to support the various device classifications) always offer the user the opportunity to change the selection.

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.

3.7 Further Considerations

The Mobile Web Best Practices Working Group would like to draw the attention of Web application developers on the following considerations. These considerations should not be viewed as best practices, because they are not prevalent in the development community at the time of writing. They are published as advisory notes.

3.7.1 Consider Use of Canvas Element or SVG For Dynamic Graphics

3.7.1.1 What it means

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 specifies a display region where JavaScript can be used to draw simple graphics. In contrast, SVG is an XML language for defining vector graphic elements that are added to a DOM which can be modified 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 better suited where a static bitmap is sufficient (e.g. drawing a scatter-chart or creating visual effects).

If speed is important, canvas may be more effective. 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.

3.7.1.2 How to do it

See the Canvas Element in the HTML5 specification [ HTML5 , section 4.8.10] for information on how to use the Canvas element.

See W3C Scalable Vector Graphics (SVG) home page [ SVG ] for information on how to use SVG.

3.7.2 Inform the User About Automatic Network Access

3.7.2.1 What it means

Network traffic on a mobile device depletes the battery and may incur expense -- so it is important to inform the user when accessing the network. Whenever an application makes asynchronous XHR data requests, whether in response to a user action or automatically, this should be indicated in an appropriate manner so that the user remains informed and in control.

3.7.2.2 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:

• how often the application will interact with the Internet - e.g. every 5 minutes, hourly, daily;
• for how long the automatic behavior will continue;
• how heavy the overall usage is expected to be or the type of service plan recommended.

3.7.3 Provide Sufficient Means to Control Automatic Network Access

3.7.3.1 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 should be provided.

3.7.3.2 How to do it

All applications that access the network automatically should 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.

Appendix 1: Best Practice Dependent Device Properties

The following device properties included in the DDR Core Vocabulary [ DDR-VOCAB ] 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:

• Display Width, Display Height, Display Color Depth
• Input Devices
• Markup Support
• Stylesheet Support
• Image Format Support
• Input Mode Support
• Cookie Support
• Script Support

Appendix 2: References

2.1 MWI References

MWBP

Mobile Web Best Practices 1.0, Jo Rabin, Editor, W3C Recommendation, 29 July 2008 (see http://www.w3.org/TR/mobile-bp/ )

2.2 Device Independence

DCODI

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/ )

DIGLOSS

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/ )

DIP

Device Independence Principles, R. Gimson, Editor, W3C Working Group Note, 1 September 2003 (See http://www.w3.org/TR/2003/NOTE-di-princ-20030901/ )

2.3 Web, Protocols and Languages

CSS

Cascading Style Sheets (CSS1) Level 1 Specification, Håkon Wium Lie, Bert Bos, Editors, W3C Recommendation, 11 January 1999, revised 11 April 2008 (See http://www.w3.org/TR/2008/REC-CSS1-20080411/ )

CSS2

Cascading Style Sheets, level 2 CSS2 Specification, Bert Bos, Håkon Wium Lie, Chris Lilley, Ian Jacobs, Editors, W3C Recommendation, 12 May 1998, revised 11 April 2008 (See http://www.w3.org/TR/2008/REC-CSS2-20080411/ )

HTTP1.0

Hypertext Transfer Protocol -- HTTP/1.0 Request for Comments: 1945, T. Berners-Lee, R. Fielding, H. Frystyk, May 1996 (See http://www.w3.org/Protocols/rfc1945/rfc1945 )

HTTP1.1

Hypertext Transfer Protocol -- HTTP/1.1 Request for Comments: 2616, R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach, T. Berners-Lee, June 1999 (See http://www.w3.org/Protocols/rfc2616/rfc2616.html )

XML

Extensible Markup Language (XML) 1.0 (Fifth Edition), Tim Bray, Jean Paoli, C. M. Sperberg-McQueen, Eve Maler, François Yergeau, Editors, W3C Recommendation, 26 November 2008 (See http://www.w3.org/TR/2008/REC-xml-20081126/ )

2.4 Other References

AJAX-UNIQUE

Unique URLs pattern on Ajax Design Patterns Web site (See http://ajaxpatterns.org/Unique_URLs )

ARIA

Accessible Rich Internet Applications (WAI-ARIA) 1.0, James Craig et al, W3C Working Draft, 16 September 2010 (See http://www.w3.org/TR/wai-aria/ )

BONDI

OMTP Reference Implementation (see http://bondi.omtp.org/ )

CSSMQ

Media Queries, Håkon Wium Lie, Tantek elik, Daniel Glazman, Anne van Kesteren, W3C Candidate Recommendation, 27 July 2010 (see http://www.w3.org/TR/css3-mediaqueries/ )

CSSMT

Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification, Bert Bos et al, W3C Candidate Recommendation, 08 September 2009 (see http://www.w3.org/TR/CSS21/media.html )

DAP

W3C Device APIs Working Group (see http://www.w3.org/2009/dap/ )

DDR

Device Description Repository Simple API, Jo Rabin et al, W3C Recommendation, 05 December 2008 (see http://www.w3.org/TR/DDR-Simple-API/ )

DDR-VOCAB

Device Description Repository Core Vocabulary, Jo Rabin et al, W3C Working Group Note, 14 April 2008 (see http://www.w3.org/TR/ddr-core-vocabulary/ )

EXI

Efficient XML Interchange, W3C Candidate Recommendation, John Schneider and Takuki Kamiya, 08 December 2009 (see http://www.w3.org/TR/exi/ )

HTML5

HTML5, Ian Hickson and David Hyatt, W3C Working Draft, 24 June 19 October 2010 (see http://www.w3.org/TR/html5/ )

HTML5-OFFLINE

HTML5, Ian Hickson and David Hyatt, W3C Working Draft, 24 June 19 October 2010 (see http://www.w3.org/TR/html5/offline.html#offline )

JSCOMP

The JavaScript CompressorRater (see http://compressorrater.thruhere.net )

JSON-PARSE

Implementation of a secure JSON parser in JavaScript (see http://www.json.org/json_parse.js https://github.com/douglascrockford/JSON-js/raw/master/json_parse.js )

OFFLINE

Offline Web Applications, Anne Van Kesteren and Ian Hickson, W3C Working Group Note, 30 May 2008 (see http://www.w3.org/TR/offline-webapps/ )

OMA-PUSH

OMA Push (see http://www.openmobilealliance.org/Technical/wapindex.aspx )

OPERA

Opera Web Widget API (see http://dev.opera.com/libraries/widgetobject/ )

OWASP

Open Web Application Security Project (see http://www.owasp.org/index.php/Main_Page )

RFC2397

The "data" URL scheme, L.Masinter. IETF, August 1998 (See http://www.ietf.org/rfc/rfc2397.txt )

RFC3966

The tel URI for Telephone Numbers, H. Schulzrinne. IETF, December 2004 (See http://www.ietf.org/rfc/rfc3966.txt )

RFC4627

The application/json Media Type for JavaScript Object Notation (JSON), D. Crockford, July 2006 (see http://www.ietf.org/rfc/rfc4627 )

RFC5724

URI Scheme for Global System for Mobile Communications (GSM) Short Message Service (SMS), E. Wilde and A. Vaha-Sipila. IETF, January 2010 (See http://www.ietf.org/rfc/rfc5724.txt)

SVG

W3C Scalable Vector Graphics (SVG) home page (see http://www.w3.org/Graphics/SVG/)

UAPROF

Open Mobile Alliance OMA-TS-UAProf-V2_0-20060206-A User Agent Profile Approved Version 2.0 06 Feb 2006 (See http://www.openmobilealliance.org/technical/release_program/docs/UAProf/V2_0-20060206-A/OMA-TS-UAProf-V2_0-20060206-A.pdf)

WCAG20

Web Content Accessibility Guidelines 2.0, B. Caldwell, M. Cooper, L. Guarino Reid, G. Vanderheiden et al, W3C Recommendation 11 December 2008 (see http://www.w3.org/TR/WCAG20/)

WEBAPPS

W3C WebApps Working Group (see http://www.w3.org/2008/webapps/)

WIDGETS

Widget Packaging and Configuration, Marcos Cáceres, W3C Working Draft, 5 October 2010 (see http://www.w3.org/TR/widgets/)

WTAI

WAP Forum wap-268-wtai-20010908-a Wireless Telephony Application Interface Specification (See http://www.openmobilealliance.org/tech/affiliates/LicenseAgreement.asp?DocName=/wap/wap-268-wtai-20010908-a.pdf)

XHTML-Basic

XHTML Basic 1.1 - Second Edition, Shane McCarron, Masayasu Ishikawa, Editors, W3C Recommendation, 23 November 2010 (See http://www.w3.org/TR/2010/REC-xhtml-basic-20101123/)