Safari browser support for offline web apps
iOS Safari doesn't currently support offline browsing for web apps. However, Apple is currently working on improving support for progressive web apps, more importantly Service Workers. 

Service Workers power offline functionality, push notifications, background content updating, content caching, and a whole lot more.

Here is a somewhat recent article on Service Workers
http://appleinsider.com/articles/17/08/05/apple-may-integrate-service-workers-into-webkit-support-next-gen-web-apps-in-safari