I'd have to agree that randomly finding something on the Internet proves little. However, for a definition of a term like "non-fiction," you're not necessarily looking for an authority, but rather just something that makes sense. (Even if you found a definition in Webster's, it carries no legal weight at all. Nor does WikiAnswers.)

Your concern about changes seems valid. Maybe this could be solved by actually copying the original cited material so the original could be viewed later by interested parties? I'd hate to make the entire Internet off-limits to legal authorities just because of the potential for change.