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 it used to be nominative + genitive:
ein Becher Weines (Schiller, around 1800)
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 book you should try to get a better one. You can check the quality of a grammar book just by checking what it says about a special problem such as this special construction with a double nominative.
P.S: 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 partitive 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. Or:
Vieles von dem, was in Wikipedia über Grammatikbegriffe gesagt wird, kann man nur als verunglückt bezeichnen.