Yeah, I don't personally mess with anything but Chrome, and I consider supporting browsers that are not Chromium-based to be a fork of the entire project.
LOTW is developed in the crouton environment, which involves ChromeOS in developer mode. All development and testing is currently done on a Chromebook, using an up-to-date Chrome browser.
The system should basically work in any modern browser and host OS, but there are likely many tiny glitches that degrade the user experience in other browsers and/or operating systems.
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The crucial fact of LOTW is that it is based around the concept of a full-featured, sandboxed file system in your browser. Only Chromium-based browsers natively support that kind of thing via 'webkitRequestFileSystem'.
Funny, that, seeing how M$ themselves are perhaps the biggest contributors to the current browser codebase monoculture issue.
Anytime a software project develops enough complexity, there is no shame in targeting a specific platform. Given enough eyes on a thing, though, those kinds of "supported" issues always take care of themselves.
But I'm just a lone developer trying to do something never before done. I'm still trying to prove a concept...
I too am a lone developer but (not to judge) I consider any website I work on to only be complete if it works on Chrome and Firefox and maybe Edge. I mean, it's easily thousands of times easier to do this now than it was in the early 2000's when we were all forced to do IE5/6/7/8 compatibility, and the tooling wasn't remotely as good... so I won't have any sympathy for you LOL
> Unhandled Promise Rejection: ReferenceError: Can't find variable: webkitResolveLocalFileSystemURL