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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.
Since 2017, the main approach for visual diffs in MediaWiki is the visual diffs system (pictured) provided by the VisualEditor extension, which compares the HTML of two documents. It provides visual diffs in VisualEditor's editing preview, as well as (still as a beta feature) in MediaWiki's standard diff display.
Previous attempts at visual diffs in MediaWiki include:
- 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.
- On-Article Diff Highlighting was an extension that was apparently under development in 2011, though no code was ever published.
- Wiki Edit History Analyzer was another apparently-abandoned attempt at visual diffs; its development was discussed in the 2010 paper "What Did They Do? Deriving High-Level Edit Histories in Wikis".
- As part of Google Summer of Code 2008, guyvdb (mentored by Brion Vibber) created an HTML diffing tool in MediaWiki core, which was later removed in 2009.