Here is a 2010 tutorial by htmlrocks.
I added app cache to my app and noticed FireFox started asking for my permission to store data.
This made me notice that other big sites like Facebook and Twitter do no use HTML5 Application Cache. In fact, I can't think of another site that has asked me for permission to store data. I'm guessing that this is because other sites do not use it.
Because it has been around so long I'm wondering if it is relevant or not ever really going to fly?
Here is the caniuse.
Update:
Removed from my app.