Pixel sizes are relative to the computer monitor, but they're not
relative to the base font of the page. For example, font sizes
designated by pixel will look smaller on a 96dpi monitor (most Macs)
than on a 72dpi monitor (most PCs). One thing I find is that when font
sizes are declared in pixels, it is not easy to resize the text, because
it must be, say, 9px. But at least if it's done in a stylesheet, you can
set newer browsers to override styles with your own personal styles.

I almost always use em sizes, which are easily resized by browsers. YMMV
of course.

-----Original Message-----
From: Alan Cantor [mailto:[log in to unmask]] 
Sent: Monday, October 28, 2002 10:18 AM
To: [log in to unmask]
Subject: Question about absolute vs. relative sizes


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]