[css-color-adjust-1] Add in Amelia's proposal. Mark alternatives as issue #4175.

Author: fantasai

css-color-adjust-1/Overview.bs

@@ -386,6 +386,10 @@ Properties Affected by Forced Colors Mode {#forced-colors-properties}
 		text-shadow: none !important;
 	</pre>
 
+	<div class="issue">
+		<a href="https://github.com/w3c/csswg-drafts/issues/4175">Issue 4175</a>
+	<dl>
+	<dt>Option A<dd>
 	For elements that have a background specified in the UA style sheet,
 	the background color is forced via:
 	<pre>
@@ -396,6 +400,17 @@ Properties Affected by Forced Colors Mode {#forced-colors-properties}
 	by forcing all channels other than alpha
 	to match ''Canvas''.
 
+	<dt>Option B<dd>
+	Based on its resulting [=computed value|computed=] 'color' value,
+	the [computed|used] 'background-color' of an element is forced
+	by matching all channels other than alpha
+	to the appropriate forced background color;
+	which should be the corresponding system background color
+	if 'color' is a [=system color=],
+	and ''Canvas'' otherwise.
+	</dl>
+	</div>
+
 	Additionally, on <em>on user input controls (except button-like controls) only</em>:
 	<pre>
 		background-image: none !important;