[css-color-4] mark system color equivalences as at-risk

Author: svgeesus

css-color-4/Overview.bs

@@ -24,8 +24,13 @@ Inline Github Issues: title
 Default Highlight: css
 WPT Path Prefix: css/css-color/
 WPT Display: open
+At Risk: Equivalence of deprecated and un-deprecated system colors
 </pre>
 
+<!-- see wpt results
+  https://wpt.fyi/results/css/css-color?label=master&label=experimental&aligned&q=css%2Fcss-color%2Fdeprecated*
+-->
+
 <pre class=biblio>
 	{
 		"TIFF": {