Visual diffs

A visual diff compares and shows differences between rendered (or displayed) documents, as opposed to showing the text (or markup) that is used to format such documents.

There are several "visual diff" efforts in the MediaWiki world:
 * As part of Google Summer of Code 2008, guyvdb mentored by Brion created an HTML diffing tool in MediaWiki core, which was later removed in 2009.
 * Extension:On-Article Diff Highlighting (2011)
 * As part of their testing efforts, the Parsing team created the "Visual Diff Testing" tool (released around April 2016), which is a pixel differ of the page rendered as an image, used to identify differences between Parsoid and the legacy parser.
 * Inside the visual editor is a visual diffs system essentially comparing the HTML of two documents (released around March 2017); eventually, this will be available on MediaWiki history and diff pages too.