[selectors] L4 comments

Section 12.4.3

   :first-child is

     an element that precedes all of its siblings (if any)

   but browsers do not implement that, they implement

     an element that precedes all of its sibling elements (if any)

   Same issue for :last-child and :only-child

Section 12.2

   Is an element containing only shadow elements empty?

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