[css-color-6] Prepare prose for multiple contrast algorithms, see #7357

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

css-color-6/Overview.bs

@@ -86,14 +86,20 @@ 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 to use.
+	The <dfn><<target-contrast>></dfn> argument specifies the contrast algorithm(s) to use.
 	If no [=color candidates=] have been provided,
 	<<target-contrast>> may be omitted,
 	in which case a UA-chosen algorithm is used.
+
 	Arguments to a <<target-contrast>> [=functional notation=]
 	indicate the <dfn>target contrast level</dfn>.
+	Multiple contrast algorithms with [=target contrast levels=] may be specified,
+	in which case all their requirements are simultaneously applied.
+	A ''contrast-color()'' function specifying multiple algorithms
+	must specify a [=target contrast level=] for each algorithm,
+	and is otherwise [=invalid=].
 
 	If the [=target contrast level=] is omitted,
 	the color candidate with the greatest contrast is returned.
@@ -120,9 +126,13 @@ If there is a target contrast</h4>
 	Candidate colors are tested sequentially,
 	starting with the first color in the list,
 	and ending with an automatically appended ''white, black''.
-	The first color to pass the specified level of contrast
+	The first color to pass the specified level(s) of contrast
 	against the [=base color=] wins.
 
+	If no candidate color passes, then whichever of ''white'' or ''black'' has the highest contrast
+	according to the first specified algorithm wins.
+	If they both have the same contrast, ''white'' wins.
+
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