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ISSUE-270 (CT and direct choice of user experience): Content Transformation on web sites that offer the user direct choice of representation. [Content Transformation Guidelines]

From: Mobile Web Best Practices Working Group Issue Tracker <sysbot+tracker@w3.org>
Date: Fri, 18 Jul 2008 12:10:55 +0000 (GMT)
To: public-bpwg@w3.org
Message-Id: <20080718121055.AA9E0C6DBD@barney.w3.org>

ISSUE-270 (CT and direct choice of user experience): Content Transformation on web sites that offer the user direct choice of representation. [Content Transformation Guidelines]


Raised by: Fran├žois Daoust
On product: Content Transformation Guidelines

We already discussed that during the F2F and several CT calls.
The issue applies to the Content Transformation Guidelines document.

In the Mobile Web Best Practices, we recommend that servers provide the user with the ability to select among different choices of presentation:
 "Where a choice of presentations is available, it is good practice to record the user's preferences and to allow them to be changed."

In the Content Transformation Guidelines, we state that the user may make a choice of user experience for a given Web Site.

The problem arises because the user may express the same kind of preferences at two different levels: the CT-proxy and the origin server. A CT-proxy cannot tell that a user made a choice of representation at the server level. An origin server cannot tell that the user made a choice of representation at the CT-proxy level.

The preferred place for handling different representations is at the origin server, since it is the one that knows for sure what it intends to return.

If at all possible, we would thus need to find a mechanism by which a server may indicate that it handles the choice of representation directly with the user, or alternatively that the user made a specific choice of representation.

Given that the semantics of the <a> element are not the same as those of the <link> element (namely the "media" attribute cannot be defined on <a> elements), there is no way we can decorate the links to the choices of representation so that they can be understood by the CT-proxy. Besides, the choice may typically be given on the home page, but not be given in subsequent pages.

This leaves us with the link element, typically used as defined in the second usage in, and the Cache-Control: no-transform directive that the server could use to say "I'm handling the user's choice!"


Actually, after further thoughts, I am not sure I understand where the trouble is. The user's choice of representation at the CT-proxy level is not exactly one, ie it's not "I want the desktop/handheld version", but rather "pretend I am a desktop/handheld browser" and it is based on altering HTTP headers, as opposed to the user choice of representation at the server level which is indeed "I want the desktop/handheld version", and it is not based on the value of any HTTP header.

Apart from the fact that the user might get lost between the two levels of choice, I do not see any conflicting point here. And not to get the user lost is up to the CT-proxy vendors, I guess.

In other words, a user may say:
 "Dear CT-proxy, pretend I am a desktop browser"
 AND "Dear server, I want the handheld version"
.... and I don't see why this would yield a problem.

If the point we want to make is that a CT-proxy should not transform a page returned as the result of a user choice of representation at the server level, then I guess we should simply advise the server to add a Cache-Control: no-transform directive to the response.

I suppose I am missing something important here. What is it?

The latest Content Transformation Guidelines draft contains two editorial notes regarding that topic:
- in section

- in section 3.2.3:

.... and I basically suggest we stay silent on this.
Received on Friday, 18 July 2008 12:12:37 UTC

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