I can imagine situations where somebody gives a link to the solution of a problem (say, some teacher's answer key, or an archive of solutions to some IMO or putnam problems) and then the link becomes broken later. Should posted answers be self-contained, e.g. either no external links, or if there are external links, it's only to information that can be easily recovered e.g. with a google search if the link breaks (e.g. a link to a page giving a proof of the A-G inequality)? And if a broken link is discovered, how should it be handled?
Links to external information are fine, but as much information as possible should be incorporated into the local answer in case the given link goes stale.