The MediaWiki software is developed collaboratively by hackers from all around the world. To help facilitate this process, standards of desirable practices have been established. While policies describe what developers must do, development guidelines typically describe what developers generally should do. There can be no deviations from policies dealing with such important topics as security for developers, even in MediaWiki extensions; but minor deviations from, say, coding conventions are often allowable in extensions. Whether there is good reason for a given deviation that is worth the downsides of conformity to the established norms is a matter for judgment of developers and their code reviewers.