[selectors] :first-node/:last-node/:only-node pseudo-classes

[LeaVerou]

Currently, :*-child pseudo-classes only consider element children, and will match even if the element has text nodes before it. For example, in <button>foo <i>A</i> bar</button> :first-child, :last-child and :only-child will all match. For many use cases, it is important to match when the node is actually the first/last/only one.

I have not found use cases requiring :nth-node(), so I think the three titular pseudos should do it.

[Loirooriol]

See #3216

[LeaVerou]

Thanks! Closing this then.