This plugin automatically and periodically saves the "just typed comment" so no data will be lost even if the browser crashes.

shawfactor

This plugin can greatly improve the user experience for your blog, by automatically and periodically saving fields like “just typed” comment (the content textarea input box in comment section), the visitors email, and vitors name (in the comment form).
The comment is saved using client side indexeddd (falling back to localstorage), this prevents loss of unsubmitted content.
This is similar like WordPress can remember your Name, Email, Website in comment area for the next visit, but WP Local Storage plugin extends this into next level, it can remember the actual unsaved comment (i.e. before hit submit button) so even the browser is crashed before the user hit save, it is okay! When the user came back, all “just type comment” will be still in there.
More importantly all data is saved client side, nothing is sent to the server. Thus maintaining visitor privacy!

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