"eine Tasse Kaffee" is a cup of coffee in English. Where most languaguages use a partitive genetive German has a special construction nominative + nominative. The first noun describes the quantity a cup,
a bucket, a sack, the second noun describes "of what": coffee, water, flour.
Today the construction for "how much of what" is nominative +nominative. But in older language it was nominative + genitive:
Schiller, around 1800: ein Becher Weines
As feminine nouns have no genitive ending it was "ein Glas Milch" and it was clear from the meaning of the words that the first noun described "how much"and the second noun "of what".
So one learned that the genitive ending of the second noun was not necessary and in the course of time it was dropped.
If you can find nothing about this problem in your grammar you should try to get a better one. You can check the quality of a grammar just by checking what a grammar says about a special problem such as this special construction with a double nominative.
PS Don't consider Wikipedia as an authority in languages. They compile a lot of things and often they don't get it right. As you see German had a partitve genitive (for nouns of masculine and neuter gender), but the genitive ending was dropped. But there are a lot of expressions where a partitive relation is expressed with "von": Tausende von Büchern/ Tausende Bücher. Oder: Vieles von dem, was in Wikipedia über Grammatikbegriffe gesagt wird, kann man nur als verunglückt bezeichnen.