I build a small webside running on the arduino with ethernet I/O using HTML and a little bit of css.
When sending a HTTP request the script reads the request and gets a command from it.
You don't need internet, only a working network and a computer of smartphone connected to the same network.
Step 1: the parts
1 ethernet shield, or a Sparkfun Pro Ethernet (not for sale anymore....)
1 LED connected to digital pin 9 and GND,