I'm thinking of using the user's default browser, not downloading another one for my program. In most cases the default browser is already running - no need to launch another instance like electron.
By using the user's browser, you do have to handle all of the different possible browsers somehow, even if that's only to do annoying browser detection and say "use a different browser". Electron and similar ideas at least limit you to designing / developing / testing a single browser. At least AFAIK; I haven't done any Electron work.
I mean... it's no different than making any website work in multiple browsers. Which really isn't too hard nowadays unless you're using bleeding-edge features or obscure non-standard ones.
Supporting multiple browsers is always somewhat more difficult than just one browser.
You are right that your choice of features plays into it, but I don't believe that's all there is. Even in the best case (all of features you want are available in all the browsers you care to support), you still should be testing in those browsers (ideally on all of the platforms / devices you support).
You also have to make the initial decision about what browsers/platforms/devices you support in the first place, and then choose when to re-evaluate your support. None of that is free.
Please don't get me wrong: I think it's a fine path (and one I have chosen myself), but it's definitely not without downsides.
I suppose you could, but I think you'd need to write version detection for a browser, the ability to figure out the user's chosen browser, the logic to download a reasonable version of Chromium, install it, get it past virus scanners, etc.
