[css-transforms-2] Paint containment causes flattening

See w3c#6202

Author: frivoal

css-transforms-2/Overview.bs

@@ -810,6 +810,9 @@ The following CSS property values require the user agent to create a flattened r
 * 'mask-image': any value other than ''mask-image/none''.
 * 'mask-border-source': any value other than ''mask-border-source/none''.
 * 'mix-blend-mode': any value other than ''mix-blend-mode/normal''.
+* 'contain': ''contain/paint'' and any other property/value combination that causes [=paint containment=].
+	<span class=note>Note: this includes any property that affect the [=used value=] of the 'contain' property,
+	such as ''content-visibility: hidden''.</span>
 
 The 'perspective' Property {#perspective-property}
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