Rationale for the warning "Same colors for color and background-color in two contexts"?

What is the rationale for issuing the warning "Same colors for color and 
background-color in two contexts"? It seems that the warning is always 
issued, with default settings, when one rule assigns the same value to 
background-color as one rule assigns to color, e.g.

a { color: black; }
b { background: black; }

I tried to find an answer in the list archives but found nothing. No 
argument whatsoever; what came closest is this:
"The validator is issuing warnings for a situation that, in a general
case, would not be desirable. If you are sure it is OK, just ignore
the warnings."
http://lists.w3.org/Archives/Public/www-validator-css/2007Feb/0006.html
But that doesn't say at all _why_ it would be undesirable. How is anyone 
supposed to be sure that it is OK if he has no idea why the warning has 
been given?

Past discussions have often coupled this with the warning about setting 
color but not background-color or vice versa (not issued by default, 
only when you ask for "All warnings"). And that's a complicated issue, 
with some known rationales, but it's a completely different issue.

So why is it regarded as potentially risky, even so that a warning is 
issued by default, to use the same color as content color in one element 
and as background color in another? Does any W3C recommendation suggest 
that there is something at stake here?

-- 
Yucca, http://www.cs.tut.fi/~jkorpela/

Received on Tuesday, 4 October 2011 19:50:56 UTC