Hello everyone.
Sorry if my question was answered earlier (I haven't find how to search on forum).
Is Diecimila board supported by WEB Editor?
Yes. For some reason they changed the board name from "Arduino Duemilanove or Diecimila" to "Arduino Duemilanove" but you should be able to use it in the Arduino Web Editor by selecting "Arduino Duemilanove" and then "Flavours: ATmega168".