Created attachment 94749 [details]
Generated content with a selector equivalent to "summary::before" renders before the expand/collapse triangle for the parent <details> element. Given that <summary> contains the text that is to be displayed after the triangle, it would probably be more correct to render it directly before that text (i.e., after the triangle) instead.
Cf: "li::before" is also rendered directly before the contents of the list items, not before the list item bullet or ordinal.
Does shadow DOM need a way to specify where the :before/:after content goes? (Something like <content> that can select :before or :after?)
In this particular case, our implementation needs to start using CSS3 Lists ::marker semantics for details marker (http://www.w3.org/TR/css3-lists/). This is, of course, is blocked in implementing CSS3 Lists spec.
Tree reforming: Unblocking 72352 because 59827 is already blocking it.