Hi folks! According to the Git log, on December 19, 2004, the 'live preview' functionality made its first appearance in MediaWiki core. (for the record: commit c05eeb66755f74272b4a5f82acc6caaeafc0fb54 https://git.wikimedia.org/commit/mediawiki%2Fcore/c05eeb66755f74272b4a5f82acc6caaeafc0fb54, Special:Code/MediaWiki/6709 https://www.mediawiki.org/wiki/Special:Code/MediaWiki/6709) Imagine how much time it has helped saving since then!
Related note: As you may have noticed, its code is currently undergoing a major refactoring. If you can, please take a moment to triage its bugs https://phabricator.wikimedia.org/T41272 and/or help drafting this RFC https://www.mediawiki.org/wiki/Requests_for_comment/Live_preview.
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