Define :only-child behaviour

Author: jakearchibald

css-view-transitions-1/Overview.bs

@@ -212,6 +212,8 @@ urlPrefix: https://html.spec.whatwg.org/multipage/rendering.html; type: dfn;
 	When a [=pseudo-element=] |pseudo| is part of a [=pseudo-element parent=] |parent|'s [=pseudo-element parent/children=],
 	then |pseudo|'s [=originating pseudo-element=] is |parent|.
 
+	If a [=pseudo-element parent=]'s [=pseudo-element parent/children=]'s [=list/size=] is 1, then '':only-child'' selects [=pseudo-element parent/children=][0].
+
 	<div algorithm>
 		To get the <dfn for="pseudo-element parent">descendants</dfn> of a [=pseudo-element parent=] |parent|, perform the following steps.
 		They return a [=/list=] of [=pseudo-elements=].