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RE: BIG ISSUE -- re Delivery Units

From: Gregg Vanderheiden <gv@trace.wisc.edu>
Date: Thu, 9 Feb 2006 13:56:44 -0600
To: <w3c-wai-gl@w3.org>
Message-ID: <010601c62db2$f3db9340$ee8cfea9@NC6000BAK>

Hi Bruce

Here is the DI group definition of perceivable unit

You have it at the wrong end of the chain.

perceivable unit
    The result of a user agent rendering the contents of a delivery unit.
User agents may or may not render all information in a delivery unit. In
some cases, a single delivery unit may be rendered as multiple perceivable
units. For example, a single html file that is rendered as a set of
presentation slides. Most perceivable units contain presentation and the
means for interaction. However, for some devices such as printers, a
perceivable unit may only contain presentation.


 -- ------------------------------ 
Gregg C Vanderheiden Ph.D. 
Professor - Ind. Engr. & BioMed Engr.
Director - Trace R & D Center 
University of Wisconsin-Madison 

-----Original Message-----
From: w3c-wai-gl-request@w3.org [mailto:w3c-wai-gl-request@w3.org] On Behalf
Of Bailey, Bruce
Sent: Thursday, February 09, 2006 1:04 PM
To: w3c-wai-gl@w3.org
Subject: RE: BIG ISSUE -- re Delivery Units

I am responding to a few threads in this single message.

Clearly I am not quite parsing the difference between Web Unit and
Perceivable Unit.  Could a Perceivable Unit (e.g., typical frame based web
site) contain multiple Web Units?  Is a strict hierarchy applicable? 

Content >= Perceivable Unit >= Web Unit >= Delivery Unit

> That works well technically but would an interactive movie be viewed 
> by many as a [web] document?

That is, I would argue, an easier lesson that teaching what is a "Web Unit".
Document has less paper connotation than page at least, and people have
dissociated Web Page from hard copy.

> Putting perceivable unit back in you get [snip]

My point is that the current definition of content needs to be seriously
reworked, since just substituting Web Unit for Delivery Unit doesn't work.
You can't include Perceivable Unit in the definition of "content" without
defining Delivery Unit.  Will Delivery Unit still appear in the Glossary?

I respectfully suggest that the challenge, and solution to this Big Issue,
is to define content without referencing Web Unit.

>> Structure:
>> 1. The way the parts of content are organized in relation to each 
>> other and; 2. The way content is organized.

> I like it.  We should consider it.  Looking at it closer, I'm not sure 
> I understand the difference between 1 and 2.
> Can you explain?  Or suggest better wording for #2 so it is clear?

I just substituted content for Web Unit from your most recent definition for

A longer definition we are kicking around here (borrows from other sources):
Structure:  Includes the hierarchical arrangement of the content and other
relationships between document elements.  Structural elements convey
organizational meaning beyond typographical formatting.  Examples of
structure include the default order of paragraphs, cross-references, and the
correspondence between header and data cells in a table.  Examples of
structural elements include headings, lists, footnotes, table of contents,

> Also - the rule doesn't limit its scope to your scope.
> The way you wrote it, it was to all content.
> Not just content within a claim. 

It is not clear to me that it is sensible to consider content that is beyond
the scope of a conformance claim.  My shop test web sites and software
against the 508 Standards multiple times a week and the issue of what it is
that we are evaluating never comes up.  Along that line, what it is exactly
we are passing or failing is also not ambiguous.

> Hmmmm example. 
> The stop sign is used in one location to indicate the control for 
> marking something as not done.  In another it is used to mark things 
> that are forbidden or don't work.
> A site may have a hundreds of thousands of pages and thousands of 
> authors.  Requiring that they all do things exactly the same is not 
> realistic or possible.

Good example, but not one that would be likely to be a problem.  There may
be multiple stop sign images, used in a variety of ways, but even ones that
are nearly visually identical almost certainly point to different file /
directory path names.  If the large sight is tightly controlled, exactly the
same image *could* be used perfectly consistently.  If the site is looser,
then there are probably multiple claims for WCAG 2.0, and therefore multiple
instances of scoping the applicable content.

Tying these recent threads together, here's a quick cut at the SC that
currently use Delivery Unit.  They are re-written here so as to avoid Web
Unit as well.  Only 2.4.4 was a stretch.

2.2.2:  Content does not blink for more than 3 seconds, or a method is
available to stop any blinking.

2.4.2:  More than one way is available to locate content where not the
result of, or a step in, a process or task.

2.4.3:  Repeated blocks of content are implemented so that they can be

2.4.4:  Content has titles wherever applicable to the baseline technology.

2.4.5:  Each programmatic reference to other content is associated with text
describing the destination.

2.4.7:  When content is navigated sequentially, elements receive focus in an
order that follows relationships and sequences.

2.4.8:  Information about the user's location within content is available.

3.1.1:  The primary natural language or languages of content can be
programmatically determined.

3.2.3:  Navigational mechanisms that are repeated occur in the same relative
order each time they are repeated unless a change is initiated by the user. 

3.2.4:  Components that have the same functionality are identified

4.1.1:  Content can be parsed unambiguously and the relationships in the
resulting data structure are also unambiguous.
Received on Thursday, 9 February 2006 19:56:56 UTC

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