Extension:RPED

In the experimental release
It causes wikilinks to turn blue and link to a remote wiki if the page title exists in a table of page titles present on a remote wiki (e.g. Wikipedia).

In the production-level release
This extension maintains a local copy of a centralized list of names of pages existent on Wikipedia; when parsing a page, wikilinks are colored accordingly. It can also act a server providing the full list and differential updates to it to other wikis that request it. The configuration of the extension can be controlled through a special page. See Extension:RemotePageExistenceDetection/Development notes for more details.

In the experimental release
An authorized user (i.e. a sysop) uses Special:RPED to upload a file of page names. See Extension:RemotePageExistenceDetection/Proof of concept.

In the production-level release
An authorized user activates the extension through a special page; as long as that browser window is open, the extension will continue downloading differential updates from the centralized list or gathering page creation and deletion data from Wikipedia's NewPages and deletion log. See Extension:RemotePageExistenceDetectionCentralizedListServer.

Installation
See Extension:RemotePageExistenceDetection/Proof of concept.

Code
See Extension:RemotePageExistenceDetection/Proof of concept.

Remaining tasks
There is still much to accomplish:
 * ✅ Limit the special page Special:RPED to admins only
 * Provide a more efficient means of importing page name files into the table
 * Provide a means of aborting imports without having to manually drop the control table
 * Provide a means of adding/removing names from the page name table via API
 * Implement green links (rather than just blue)
 * Provide a different method of differentiating "green" links from "blue" (e.g. different font) for accessibility
 * Provide translation into other languages
 * Enable extension to act as a server for the page name list
 * Enable downloads of page name lists from other wikis with the extension