You're probably misreading; it's almost certainly HTML5 ("five"), not "s." I can't imagine any employer would ask for experience with all of those other languages and NOT ask for HTML5. And the fact that HTMLS does not exist is another clue ;-)
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I copied and pasted the quote straight from Craigslist. I think Daz is correct it might represent Server Side Includes. A quick look on the internet turned up that htmls file are server side files. The job is also for my local office of education, and schools have been known for archaic technology. I guess I'll know more on Friday.