[karterk]

Offline first is almost magical for end-users, since this is something quite contradictory to how people perceive the web. Having said that, it's also incredibly difficult to get it right.

As others have pointed out, the major problem I face is to inform the users that some features are not available when they are offline (e.g. file uploads). If you're not careful with the user experience, this leads to lots of support headache.

Another problem is dealing with legacy client data. If you are storing anything at all in the local storage, you need to realize that it's going to be always there once you write it. So handling with data migrations on the client storage becomes very important, and definitely must be thought ahead.