I have an existing HTML5 front-end to an MMORPG project that was started but not yet completed that could potentially be repurposed to your requirements. The system includes server-side game logic (which could be useful if you want to implement a cash shop for game items, etc) that could easily be modified for single player games. It is quite closely tied to the 'tiled' map editor, so you would need to be comfortable using that tool to develop your content. Let me know if you are interested and I can provide more details.