Possessive s

[konungursvia]

I follow Strunk & White on this; I like their concision.

[5jj]

The omission of the 's' may well be more common in AmE. It is, however, clearly not a point worth arguing about.

It may not be worth arguing about, but the Corpus of Contemporary English suggests that the omission of the apostrophe is far less common in American English.