We need a way to register custom extensions from localsettings.js. These should provide handlers that work on the token stream (by emitting tokens) or on the DOM (by converting the extension tag into an HTML DOM), and in the serializer (to convert an extension node back to wikitext).
The details are hashed out in https://www.mediawiki.org/wiki/Parsoid/Extension_registration_interface.
Version: unspecified
Severity: normal