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In https://dvcs.w3.org/hg/csswg/rev/829b2b0f3be8, a link to nth-child with no link text was added, along with the typo "exte adds".
That part seems to be in WHATWG CSS Books[1] now, and the typo seems to be fixed. Closing. [1] https://books.idea.whatwg.org/