Hmm, this test is also assuming that the initial value of 'flex' allows shrinking, and that the initial alignment behavior is stretching. I think it's a correct test, but maybe those assumptions should be documented. :)

Alternately, could write a test that doesn't rely on these. E.g. by setting the flex items to be inflexible and checking that the items overflow the flex container instead of wrapping. (Maybe give the container thick red right border that gets covered up by the second item.)