Extension:Parsoid

The Parsoid service, which is written in Node, can use this PHP extension to populate and co-ordinate a cache with pages so that they are available immediately for users.

Required for MediaWiki 1.23 users of VisualEditor
For MediaWiki 1.23 users, it also provides a styling module that is a dependency of VisualEditor, so this extension must be installed for VisualEditor to work on MediaWiki 1.23. From MediaWiki 1.24 onwards, this styling module was upstreamed into MediaWiki itself, and so this extension is not required unless you have a deployed Varnish cache.