General
Seriously, though. What is a progressive web app?
https://medium.com/@amberleyjohanna/seriously-though-what-is-a-progressive-web-app-56130600a093Progressive Web Apps 101: the What, Why and How
https://www.freecodecamp.org/news/progressive-web-apps-101-the-what-why-and-how-4aa5e9065ac2/Ionic Framework
https://ionicframework.com/docs/v3/developer-resources/progressive-web-apps/