, None.
.htmit was usually normal when the extension length was important (in the days of 8.3-character file names), but this was not a problem for a long time, but is .htmlmore common today because it is more descriptive. As for the website, you can use any extension that you like, or nothing at all (as long as the server is configured to send the correct mime type) - the extension is a problem only on your local computer.
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