Have you ever tried Travian? It is an online strategy game that only requires a regular webbrowser. It is pure xhtml and ajax.
The game is all about world domination, but less can do. You start with a small city that you have to develop, and as it grows larger new buildings and units will be available to you. You must also remember to harvest enough ressources to allow for continued expansion.
To succeed you need to find a couple of allies, everybody else will be your enemies. Trust no one.
And beware, the game is highly addicting.