[css-overflow] Add computed value logic for overflow and contain: paint

Author: frivoal

css-overflow/Overview.bs

@@ -28,6 +28,8 @@ type: dfn; spec:css-multicol-1; text:overflow column
 <pre class="anchors">
 url: http://www.w3.org/TR/2008/CR-css3-marquee-20081205/#the-overflow-style; type: property; text: overflow-style;
 url: http://dev.w3.org/csswg/selectors-3/#subject; type: dfn; text: subject;
+url: http://dev.w3.org/csswg/css-containment/#propdef-contain; type: property; text: contain
+url: http://dev.w3.org/csswg/css-containment/#paint-containment; type: dfn; text: paint containment
 </pre>
 	<style>
 		table.source-demo-pair {
@@ -420,6 +422,15 @@ Introduction</h2>
 				then computed values are the cascaded values
 				with ''overflow/visible'' changed to ''overflow/auto''.
 			</li>
+			<li>
+				Otherwise, if both cascaded values are ''overflow/visible''
+				and the computed value of 'contain' is
+				one that activates <a>paint containment</a>
+				(e.g. ''contain:strict''
+				or ''contain: paint''
+				or ''contain: layout paint''…),
+				then the computed values of both 'overflow-x' and 'overflow-y'
+				are changed to ''overflow/clip''.</li>
 			<li>
 				Otherwise, the computed values are as specified.
 			</li>