Architecture of a web-enabled game

Ok, so the compo starts in six hours, but I'm in the UK so I probably won't start coding until tomorrow morning.

Don't want to cheat by collecting game assets, so the only thing for me to do this evening is try to clarify my architecture.

My aim is to incorporate a RESTful web service for semantic game content, and to present multimedia locally. So, I plan to have a client in Ren'Py which plays, eg: atmospheric effects according to what it perceives as local conditions. I'm hoping to use as a web server, keeping game state there and serving human readable content as XHTML and data for client consumption as JSON.

Of course, there are grey areas. For example; how will time be communicated/synchronised between client and game world? How much does the game server need knowledge of client state, etc.

I know others have been thinking of a web-oriented approach. Has anyone got any advice to offer, or any strong opinions about architecture?