It appears that Apple may be working on adding service workers support to WebKit. The WebKit Feature Status webpage has recently been changed from "Under Consideration" to "In Development." This change may be an indication that Apple will eventually support Progressive web apps (PWAs).
At YourTrove, a lot of what we do involves ingesting social content via APIs and then regularizing that data within our system. For a lot of data, this is pretty straightforward. For example, while Facebook and Flickr might return different meta data, or name fields differently, no one disagrees that a photo is a photo. This is true for essentially all uploaded binary user generated content.