[css-color-6] Add handling for non-opaque colors in contrast-color()

Relates to #7358

Co-Authored-By: fantasai <725717+fantasai@users.noreply.github.com>

css-color-6/Overview.bs

@@ -16,7 +16,7 @@ Editor: Lea Verou, Invited Expert, http://lea.verou.me/about, w3cid 52258
 Editor: Adam Argyle, Google, https://nerdy.dev, w3cid 112669
 Abstract: This module extends CSS Color [[css-color-5]] to add contrast calculation.
 Repository: w3c/csswg-drafts
-Inline Github Issues: title
+Inline Github Issues: false
 </pre>
 
 <pre class='ignored-specs'>
@@ -86,7 +86,7 @@ Computing a Contrasting Color: the ''contrast-color()'' function {#colorcontrast
 	or as a background against a list of [=color candidate|candidate=] text colors.
 	(The effective contrast of a pair of colors depends on their usage.)
 
-	ISSUE(#7359): The keywords to specify whether the [=base color=] is a foreground or background are <abbr title="to be decided">TBD</abbr>.
+	ISSUE(7359): The keywords to specify whether the [=base color=] is a foreground or background are <abbr title="to be decided">TBD</abbr>.
 
 	The <dfn><<target-contrast>></dfn> argument specifies the contrast algorithm(s) to use.
 	If no [=color candidates=] have been provided,
@@ -272,13 +272,27 @@ If no target contrast is specified</h4>
 		* the contrast is (0.92781 + 0.05) / (0.03568 + 0.05) = <b>11.4123</b>
 	</div>
 
+<h4 id="contrast-transparency">
+Contrasting Semi-transparent Colors</h4>
 
+	This section applies only to contrast algorithms that require their inputs to be opaque colors.
 
+	When calculating the contrast of a pair of colors:
 
+	1. The background color is first blended over an opaque canvas color
+		using simple alpha compositing (see [[compositing-1#simplealphacompositing]])
+		to find an opaque background color.
 
+		<div class="issue">
+		<a href="https://github.com/w3c/csswg-drafts/issues/7358">#7358</a> Find a good opaque canvas color for this computation.
+		Candidates are:
+		- the foreground color with alpha = 1
+		- the ''canvas'' system color
+		- some other color calculated to minimize the contrast for the algorithm used
+		</div>
 
-
-
+	2. The foreground color is then blended over this opaque background color
+		to find an opaque foreground color.