When it was new, WP8.1 browser support for mobile-focused websites were pretty good. This was somewhat out of necessity: mobile developers tend to release dedicated iOS and Android apps, leaving WP users to their website, so the browser had to work. Roughly on par with competitors of its day, mobile site authors could support WP8.1 with minimal (or no) additional effort. But the web moved on and WP8.1 did not. Soon support for such browsers became an explicit opt-in that fewer and fewer people chose. With support dropping left and right, Microsoft will soon forcibly remove IE11 from existing installations of Windows.
It hasn’t been practical for several years to “just” keep a browser app project compatible with IE11/WP8.1. Even worse now that IE11 debugging resources are being removed. I still hold hope of using my old Windows Phones in a project of some sort, but it would have to be a dedicated project focused on using just the capabilities it has. It has become effectively a development platform separate from modern web development. Based on my earlier ESP32 Sawppy controller project, I know I still have access to the following: draw to screen with HTML Canvas, touch input with PointerEvent, and communication with WebSocket. This is a tiny subset of the breadth of modern web development, but enough foundation to build something neat. I have to think up a project idea and do it before all IE11-related debugging resources disappear.
In the meantime, I’m going to ignore WP8.1/IE11 compatibility for my AS7341 interactive web app. I will move forward with an improved user interface and only have to worry about how it works on my Android phone Chrome browser.