My keyboard isn't Spanish, and so I tried using https://spanish.typeit.org/. It gave the same result: errors in the latest testing because of the U+ codes used.

https://en.wikipedia.org/wiki/UTF-8#MUTF-8 (under the title 'Standards')refers to an updating of utf-8 standards in 2018. I suspect that the latest version of the Nu Html Checker is using the latest standards. Presumably the matter is outside the control of SD4, and I would be inclined to ignore the error shown, or to find some way of entering those accented capitals with the new U+ codes.

Frank