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.