HTML5 is the "old" coin, it's basically dead and abandoned (limited support by the new htmlcoin team). The only reason you should be interested in the old HTML5 is that you can swap them for htmlcoin for the next few weeks (see htmlcoin website). This means you can mine the old HTML5 using your X15 miner, and get nice and shiny new coins for it (until the swap window is over). You would need a 1.3 wallet, which are hard to find as the download links disappeared (which is probably a good thing).

htmlcoin is on a number of exchanges (see their thread or website) where you can buy them. Download the 2.x wallet from the website!