Yes there is. It's called "navigator.registerProtocolHandler"[0]

[0]: https://developer.mozilla.org/en-US/docs/Web/API/Navigator/r...

Not declarative, which means users would need to know they have to go to settings and enable it.

It shows a prompt to the user when you run the code, they don't need to go near settings.

Nope that's now how it works. It does show a prompt. Also, not sure how "going to settings" is "not declarative"...? :) ;p xx

> the only issue is that there's no declarative way to advertise support for mailto links, webmails need to ask the user.

What's the alternative? You can't let any website register a handler for mailto without asking the user

Let any website register a handler for mailto links.

Get the OS/browser to show a list of registered handlers when a mailto link is clicked.

Order the list by frequency of use so random websites you visited 5 years ago stay nicely below the actual one you want to use. Also maybe unregister or hide anything not visited for 30 days, so yo'd have to revisit the site to reregister it.

Doesn't Firefox do it already? I've just clicked on a mailto link and it asked me whether I'd like to go to gmail.com or to launch the external program... but then again, normally I don't ever click on mailto links, because I don't usually write e-mails anyway, and when I do, I just select and copy the e-mail address, then paste it into the "To:" field.

Firefox has an option for the application (or website) to open a mailto link with. It's lacking Outlook OWA though. You can find in the preferences panel where all default 'open with' settings are

Then you need to prevent any website from registering a ton of handlers, even through redirection or links to other (sub)domains.