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
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