Why does the the :last-child selector only selects the “lastest child”?

That's not true. They both select all the last and first children of their parents within the document respectively. You have either misunderstood the meaning of "last child" and "first child", or misinterpreted the structure of your page.

P:last-child selects p elements that are the last child of its parent therefore it is not followed by any sibling elements. I expect that this will have a pink background but it won't :( Why? Is followed by a div which is the last child not its previous sibling the p.

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