Parsoid is software we use for the visual editor, content translation, Flow and the Android app. This has been rewritten. It will come to the wikis gradually over the next two weeks. It has been tested, but there could be some diffs or previews that don't look right. If you see any you can report them. 
The new version of MediaWiki will be on test wikis and MediaWiki.org from 26 November. It will be on the other wikis next week (calendar). This is because of holidays.