Parsoid/Visual HTML diff

The text-based diff used in MediaWiki works ok to compare two wikitext revisions. For HTML documents however, visual diffing would however be much preferable to avoid exposing users to the verbosity of raw HTML.

A good example for such an interface is implemented in LocalWiki. Two fully-rendered versions are displayed side-by-side, with change information shown in pop-up bubbles as the user steps through the diffs. LocalWiki uses an external Java service for the diffing. A PHP version of DaisyDiff for Mediawiki has been developed as part of GSoC 2008 which has since been removed from Mediawiki core, but is available as an external module. This codebase might be a useful starting point for providing visual diffing. An alternative we might implement in Parsoid for change detection purposes is the XyDiff XML diff algorithm.