Fallback URI handlers on a webpage

Let's say you have a mastodon and a website, and you wanna put mastodon links that open in the user's mastodon on your website. So you go on your website and sprinkle web+mastodon: around only to find out that browsers are user-hostile and drop the user into a very confusing error page.

It'd be nice to have a way to set all links on a website to fallback to a specific URI handler, so that it, for example, goes to your mastodon instance, or to a mastodon tutorial.

These "fallback handlers" would have to be URI-specific tho, because you may also want some ganarchy links on the same webpage, which uses web+ganarchy URIs instead. So you'd want, web+mastodon to be handled by a mastodon instance and web+ganarchy to be handled by a ganarchy instance.

Nothing gets registered tho, so if the user goes to another webpage, they get to either use their own handlers or if none are available, that webpage's specified fallback (if any).

Start by reviewing the proposed fallback behavior for the web+mastodon and web+ganarchy URI examples in the issue. Define how a webpage selects a URI-specific fallback without registering a handler globally, and consider the behavior when users move to another webpage; the work is done when those cases are specified clearly.