The popular "Cool URIs don't change" article at <https://www.w3.org/Provider/Style/URI> says:
> What to leave out
> ...
> File name extension. This is a very common one. "cgi", even ".html" is something which will change. You may not be using HTML for that page in 20 years time, but you might want today's links to it to still be valid.
But I have been running my website for over 20 years now and I do think I'll stick with ".html" for the foreseeable future. This combined with the fact that I strictly use relative links for cross-linking between pages, for loading CSS, images, favicons, etc. means that I can browse my website offline (directly from my local disk) too just by opening the local index.html file on my web browser.