[css-contain-2] contain:style only applies if there is a principal box

See #7392

Author: frivoal

css-contain-2/Overview.bs

@@ -1061,6 +1061,12 @@ Style Containment</h3>
 	contain-style-breaks-005.html
 	</wpt>
 
+	However, giving an element [=style containment=]
+	has no effect if any of the following are true:
+
+	* if the element does not generate a <a>principal box</a>
+		(as is the case with ''display: contents'' or ''display: none'')
+
 	Note: [[CSS-REGIONS-1]] has normative requirements on how <a>style containment</a> affects regions.
 
 	A <dfn export for=property lt="scoped | scoped property | scoped properties">scoped property</dfn> has its effects scoped to a particular element or subtree.