2 #Web problems which seem like they might share a solution:

1) You link to a page, but there's a chance that it's gone offline or has a too-steep paywall, so you want to also link to an archive site like web.archive.org or archive.ph. It's possible to put those links adjacent in the HTML, but that stops the UA from doing something intelligent if it receives a 404 or the user otherwise expresses that the main link didn't work.
2) https://www.tbray.org/ongoing/When/202x/2025/04/16/Decentralized-Schemes and https://github.com/mastodon/mastodon/issues/19679#issuecomment-1301180085 you're supposed to be able to link to a scheme: url to get to the user's choice of application to handle that scheme, but this leads the user to a dead-end if they have _no_ application for the scheme.

An obvious-solution-that's-probably-wrong: Add a fallbackhref="space separated urls" attribute to link tags, and have browsers automatically load them in sequence if earlier ones "fail", for a definition of "fail" that includes at least 4xx and 5xx HTTP errors and missing protocol handlers.

Edit: Initial very-drafty explainer: https://github.com/jyasskin/handling-unreliable-links