I don't think people would've complained so much about Edge if it wasn't the monster full of adware that it is today.

I actually used Edge for a few years when it came out and it was good, but each version they shoved something new in your face and I had to go back to Chrome.

Edge used to be a legitimately valid choice

Edge was great just after they switched to Chromium and lead the way on battery optimizations. And then every team started an everlasting "who can exhaust the patience of their users the fastest and get them to either give in to microsoft-everything or switch away from edge" contest.

That makes me wonder: What are the rules for something to be considered a browser? Does it need to provide certain functionality, or can I register anything as a browser and just have links open in an editor?

Afaik to make a program selectable as default browser, one needs to register it as a protocol handler for http and https urls in the registry.

https://stackoverflow.com/a/38205984/2306536

Does that mean I can use a headless browser as default ?

You can probably put any cmd-executable command as default, including headless browsers.

I tried exactly that. Turns out it can't be a .bat or .ps1 for understandable reasons, it has to be an executable. So, my "default browser" on this machine I'm on is now an .exe that prints $1. It works.

In Windows, default applications are registered either by URL scheme (so ftp, http, https, gopher...) or file extension (.txt, .html, .pdf, .mp4...). As far as I know there is no special registration for "browsers" specifically, you just register your application as a handler for whatever protocols and file formats you want.

So if you want to make your random executable be "a browser", my guess is you should register it as a handler for http, https, and .html, at least.

To enforce the use of Edge, they're using URLs with the "microsoft-edge" scheme.

Some time ago Microsoft made it impossible to reassign this scheme to another app like EdgeDeflector.

PS: It was KB5007262 that "addresses an issue that might improperly redirect OS functionality when you invoke microsoft-edge: links."

If it's Chromium, it's a web browser. Steam? Web browser. Literally any Electron app? Web browser.