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Notjohn - have to disagree, auto-save has nothing to with Windows per-se, any more that it does with DOS or CP/M
Lot's of Windows programs have an auto-save feature. Two programs that I use in conjunction with Sigil are Word and Notepad++ - both have an autosave feature (both set to 2 minutes).
Kevin and DD have responded to - 'Could Sigil have an auto-save feature?'
But besides the issues they've raised there is also complication that an EPUB is 'merely' a bunch of files in a zip - what would be auto-saved, everything or just the file currently under edit.
BR