I said “…a dot html” in my head, but then I read his first example as “an html extension”. Thought I was looking and reading but was not sure what happened. Just a couple of hours later again. My brain skipped the.. and is not even thinking. In. in. html because it didn’t see it as ending a sentence, so it somehow didn’t mentally ‘count’.

So follow normal rules with extensions if you start with a vowel and you spell ‘an’, regardless of the preceding dot or not. But

I’d say its not useful for an extension right now.