Im looking to code a simple point of sale system using HTML as a progressive app, in other words, creating a web app that doesn't run in a browser.
How would be the best way to get an EPSON printer to communicate with the app. I've used JAVAPos API in the past for communication. Is there any way to get JAVAPos to work with a web app?
Since you specifically say that you're interested in developing something that runs outside of the browser, but you presumably want to use web technologies like HTML, CSS, and JavaScript, pursuing an approach based on an Electron container, and using a library like electron-usb to communicate with your printer, would seem like the best option.
If you're open to other options, it is possible to write a traditional web app that runs within certain browsers and talks to USB devices, using the WebUSB API.
Another approach would be to write a Chrome App that accomplished something similar, but that's deprecated if you don't plan on running on a Chrome OS-based computer.
