MWABP: Reworked text for Handling Variations in Delivery Context.

Hello all,

Having lost some momentum with so many weeks without calls I'm just trying
to get back up to speed here. Below is the current text for 3.6 in MWABP
after modifications based on the resolutions in the last meetings. As
agreed, I have removed references to DCCI and OMA DPE and expanded the text
around CSS MQ. The text on CSS MQ is a little terse, but hopefully adequate
given the context.

Comments please.

Adam.

3.6 Handling Variations 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 and/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 Possible3.6.1.1 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 transmission, thus improving the user experience and avoiding
transmission of unnecessary or incompatible data.
3.6.1.2 How to do it

In its most basic form, the minimum evidence from the client device is the
HTTP Accept header.

In practice, this evidence is insufficient to determine the key properties
that will help to select/adapt the content appropriately, so additional
headers or other sources of information should be considered. Typically, the
following headers provide evidence of device capabilities:

   - *Accept header:* this list of MIME types can aid in the selection or
   creation of alternative content representations to suit the requesting
   device. This header is not always reliable however, and its value is often
   set to “*/*”, suggesting that clients can accept every MIME type.
   - *User-Agent header:* as a generally unique (albeit opaque) string it
   can be used as a key into a device descriptions repository (DDR). The set of
   properties recorded in these repositories will vary 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 header:* 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 header is sometimes used with a DDR where corrections to the profiles
   are stored.

3.6.2 Use Client-Side Capability Detection Where Necessary3.6.2.1 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 behaviours 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) will need to 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> tag
   to the DOM or by an XHR request).

*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 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 <#CSSMQ>] for more details.