Try this:
1. Open the HTML file in a web browser (I'm using Safari for my test here, on a local HTML file I saved off a recipes website, just to confirm it works.)
2. From the Share menu, pick Paprika
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3. Paprika will launch and show the recipe import, which you can double-check, edit, add categories, etc. before it happens. Hit Save and you're done. The recipe will now show up in your Paprika database and sync over to any other machines you have it running on.
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note: If you're not seeing Paprika there, and you've installed the app, then double-check your sharing extensions (System Preferences > Extensions > Share Menu) to make sure it's activated:
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If it was me, I'd also hang on to and archive the HTML files as they're likely to stay readible for many many years to come.