Is it standard practice not to include an ‘offline’ mode for a web application?

There are many discussions around the topic of accessibility, but taking the work accessibility in its pure sense of having access to the system, I am curious as to whether designers simply make the assumption that network access is standard and therefore there is no need to create an offline mode.

The primary scenario where I have seen this being implemented is in games where you need to connect online to access community or multi-player features, so they differentiate by having a single player mode, but many still connect to the web to save data or access information.

Is this a technical consideration around being able to save and process data locally versus on the server side, or is it just a given in most use cases that network access or interruption doesn't happen? Is this actually a reasonable assumption to make or should we always try to provide an offline mode by default?

Update: Considering that the Polymer Project includes the Platinum Elements for dealing with all the caching and request handling logic, it seems like at some stage offline modes may become more mainstream.