I am by no means an expert, but It would seem to me that elements related
to the spacial layout of the screen should be created by using percentages.
 I think you might be getting confused with the dynamic relationship of
relative percentages of the screen verses the affect a screen size can have
on an element using pixcel size.  On the other hand, someone could make the
pixcel size relative to the screen pixcel size variable if one can create a
script or program to extract that information from every user agent hitting
the page.  I am not overly familiar with CSS 2.? or 1.0 for that matter.
Does it provide a utility to extract the screen size variable from a user
agent's computer?  It is just so much easier to use percentages.
As for using pixcel size for borders and similar elements, I would suggest
the element use dynamic relative measurements as well to avoid visual
distortions.  I am totally blind, and I honestly don't care about the size
of borders anyway <grin>.

At 09:18 AM 10/28/02 , you wrote:
>Hello EASI Colleagues,
>
>I would appreciate hearing comments from web accessibility experts on this
question I received from a web developer who is retrofitting a site.
>
>Alan
>
>The question:
>
>Which length measurements are relative?
>
>The WAI examples list percentages (%) and em (base font height) as
examples of relative length measurements (slide 32). BUT, the CSS1 and CSS2
>specifications also list ex (the x-height) and px (pixels)
>as relative measurements. "px" is relative because
>
><blockquote>
>Pixel units are relative to the resolution of the viewing device, i.e.,
most often a computer display. If the pixel density of the output device is
very different from that of a typical computer display, the user agent
should rescale pixel values. It is recommended that the reference pixel be
the visual angle of one pixel on a device with a pixel density of 90dpi and
a distance from the reader of an arm's length. For a nominal arm's length
of 28 inches, the visual angle is therefore about 0.0227 degrees. (CSS2 spec)
>
></blockquote>
>
>Since the pixel is defined in terms of a visual angle and a distance,
isn't it really an absolute value just like in (inches) or cm
(centimetres)? Also, it's not resizeable by the user like font-relative
measurements are, is it?
>
>On the other hand, it's very useful for defining things like border-width
or image sizes. Is it OK (i.e. accessible) to use px for certain properties
and not for others?
>
>
>
>Alan Cantor
>Project Manager
>Strategic e-Government Implementation
>e-Government, OCCS
>416-212-1152
>[log in to unmask]
>
---
Ross Eadie
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