I am in the middle of a redesign where I will be off loading the heavy lifting of the web page and all decision making to a raspberry pi. So basically you write the web page and serve it from the pi. You have a program running on the pi that communicates with the arduino over i2c.
I've got a write-up of a demo project with example code that does a similar thing distributing the "heavy lifting" part of the web-serving to a Pi, except the Arduinos connect to the Pi via a wireless link using nRF24L01+ radios instead of a wired link.
The demo project was actually inspired by wallaceb's reptile program, and although the demo application doesn't control a vivarium, the web page design will immediately trigger recognition! (Thanks again wallaceb! :-)
Anyway, here's the link to the project pages: http://embeddedcoolness.com/rfxduino-demo-projects/
Note that you can't really compile and run the code in the later stages of the sketch development without the RFXduino gateway system, but even without it the general design principles and structure of the program may be of interest.