Re: Color contrast between semi-transparent colors

Marc said: 
"You have a background and a foreground and you use a semi transparent 
layer between them. So just make sure (if text is dark) that the darkest 
possible background (black) will be still bright enough to produce a 
satisfying contrast ratio.

Or use a semi transparent layer which darkens even a white background so 
that you can use a white font. If the font should be not white, you must 
darken the background even more...

Marc"

and Lea said:
"I've started another thread about variable color backgrounds [1]. We 
cannot assume that just because the background is semi-transparent, it 
will be overlaid over a variable color background. For example, many 
designers use semi-transparent white or black on top of solid colors, to 
create lighter and darker variants while still being able to change the 
base color with one edit.

[1]: http://lists.w3.org/Archives/Public/w3c-wai-ig/2012JulSep/0329.html

Lea Verou"


The question I think that needs to be asked and answered  is who is the 
"you"?  in other words, what is or should be the role of the user agent vs 
the responsibility of the developer?
 
I believe the user agent should be able to change the middle 
"semi-transparent layer" to imporve contrast accessibility.  If user 
agents can already "turn off" images, why not also be able to place a 
'semi-transparent layer' if one doesn't exist, or re-place the one with a 
darker or lighter one.  Again my point being that desired solution belongs 
with the user agent and the responsibility of the developer is to enable 
it. 

Also, is there a grey color that at some point can be contrasted with 
either white or black background and achieve the same contrast level 
either way?  And did WCAG 2.0 minimum contrast level of 3.5 to 1 then in 
effect prohibit certain grey fonts becasue they can't achieve the minimum 
level?
____________________________________________
Regards,
Phill Jenkins, 
IBM Research - Human Ability & Accessibility Center
http://www.ibm.com/able
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