The page has a shadow root that in turn contains two shadow roots. So we can
check behaviour with both nested roots and younger/older sibling roots. The
various sections are highlighted with colored borders to make it more obvious
where each element comes from.
Base-level Stuff
Parent
Parent Contents
Stuff for for the younger child.
Older Child
As the older child of a nested shadow root, this is the most likely
to go wrong bit of the page.
Older Child Contents
Younger Child
Younger Child Contents
Younger Child Shadow
This should never be shown. The ShadowDOM is older than the parent shadowDOM,
and parent doesn't have a <shadow> element.