Visual diffs

There are several "visual diff" efforts in the MediaWiki world:
 * Inside the visual editor is a visual diffs system essentially comparing the HTML of two documents; eventually, this will be available on MediaWiki history and diff pages too.
 * As part of their testing efforts, the Parsing team created the "Visual Diff Testing" tool, which is a pixel differ of the page rendered as an image, used to identify differences between Parsoid and the legacy parser.
 * 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)