Best practices for developers

Security: https://www.drupal.org/writing-secure-code and the Sanitization functions topic
Coding standards: https://www.drupal.org/coding-standards and https://www.drupal.org/coding-standards/docs
Accessibility: https://www.drupal.org/node/1637990 (modules) and https://www.drupal.org/node/464472 (themes)
Usability: https://www.drupal.org/ui-standards
Internationalization: <a href="../../../core%21lib%21Drupal%21Core%21Language%21language.api.php/group/i18n/11.1.html" title="Internationalization and translation" class="local">Internationalization topic
Automated testing: <a href="../testing/11.1.html" title="Overview of PHPUnit and Nightwatch automated tests." class="local">Automated tests topic

Overview of standards and best practices for developers

Ideally, all code that is included in Drupal Core and contributed modules, themes, and distributions will be secure, internationalized, maintainable, and efficient. In order to facilitate this, the Drupal community has developed a set of guidelines and standards for developers to follow.

Standards and best practices that developers should be aware of include: