[css-color] Call out different behavior from Gecko

Author: frivoal

css-color/Overview.bs

@@ -186,6 +186,9 @@ Resolving Color values</h2>
 		<li>
 		The computed value and used value of ''<color>/transparent'' is ''rgba(0,0,0,0)''.
 
+		Issue: Gecko disagrees, and serializes any <<color>> with an alpha channel of 0 as transparent.
+		No other browser does that though.
+
 		<li>
 		The computed value and used value of
 		<a>named colors</a> (including <<deprecated-system-color>> colors),