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>.