The meta bug tracks implementation of prefetch part of speculation rules API.
Speculation Rules API Specification
Mozilla's Speculation Rules Strategy

Capturing some of the Chromium changes and issues that Mateusz bumped into in the hopes that we can avoid problems :)


Speculation Rules cache the non-cacheable responses:

• You need to return an error response (4xx or 5xx) to prevent that
• Link rel=prefetch used to work that way but doesn’t anymore
  Remove <link rel=prefetch> five-minute rule


Issues:
TTFB and other resource timing entries incorrect for speculation rules prefetches
Removing a link that is being speculatively loaded causes the request to hang forever
Speculation Rules Prefetch cache is used for POST requests

Let us know if you need any more info on any of those.

BTW here's another good talk on it (and Tim also ran into that last bug too):
https://slideslive.com/39043167/one-step-ahead

It feels like implementing the 'execute on mousedown' part of the conservative eagerness is probably the highest priority and most important portion of this.