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Sometimes if I just click on Unanswered then the Unanswered Tags "html" is missing in the right column, just sometimes (but mostly). if I scroll down the page I see unanswered html questions...
No, this is not a bug. Only the 25 tags with the most unanswered questions are shown. Since there are only nine unanswered questions tagged with html, it doesn't appear in the list.
By the way, I strongly encourage everyone to dig though the unanswered questions once in a while and answer one. You'll improve your reputation, the internet, and help us make progress in the beta.