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?