Why does GROUP BY work?

Question

SQL

According to the wiki (and others) the code below shouldn't work, but it does. Why?

SELECT title, genre, MAX(checkouts) FROM library_books
GROUP BY genre;

Checked here
In MySQL, it does not work correctly, but also without an error.

Answer 1

will not work in PostgreSQL
What works is the quirks of the DBMS

Answer 2

Why shouldn't it work?
Should, just what result you expect.
The response from the database was received, but in the specified example, the values ​​were glued into one line (grouped).

('first', 'comedy', 5),
('second', 'comedy', 7),

and eventually got 2 results as a result of the sample. Without grouping would get 3