Manual:$wgOOUIEditPage

The edit form since MediaWiki 1.29 uses pretty OOjs UI buttons, checkboxes and summary input. Because this change can cause problems for extensions and on-wiki scripts depending on the exact HTML, the old version is still available and can be used by setting $wgOOUIEditPage = false; in LocalSettings.php. This variable will be removed later and OOjs UI will become the only option.

To make testing easier, users can also force either mode by adding &ooui=true or &ooui=false to the action=edit URL.