The MediaWiki software is developed collaboratively by people from all around the world. To help facilitate this process, standards of desirable practices have been established.
Development policies describe what developers must do. There can be no deviations from policies.
Criteria and process for PHP version support in MediaWiki core and components
Code review rights and responsibilities for MediaWiki core and components
Annotation conventions and deprecation process for stable interfaces in MediaWiki
Policy for MediaWiki database queries and database schema changes
Development principles and practices for Wikimedia engineering projects
Principles for developing and deploying a Wikimedia service
Unlike policies that describe what developers must do, development guidelines describe what developers generally should do. Minor deviations from guidelines, like coding conventions, can often be allowed based on individual circumstances. 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.