All it says at the Manual of Style, as far as authority is concerned, is:

If this page does not specify which usage is preferred:

 Use other reliable resources as style guides, such as The Chicago Manual of Style (from the University of Chicago Press) or Fowler's Modern English Usage (3rd edition) (from the Oxford University Press).

In my opinion, Fowler's 3rd edition is weak: most of Fowler's original entries have been replaced by Burchard's dreary ones, which sit so much on the fence as to be little help in resolving disputes. I'm embarrassed to say that I have more usage guides in the house than is healthy in a normal man and have read several of them all the way through, so if you do need something looking up in Chicago or whatever, just let me know. (If you quoted Chicago to someone who was arguing with you, you could pretty well say that you had the backing of Wikipedia's style policy, I'd say.)