[theandrewbailey]

Yes, they are interchangeable.

On my blog ( https://theandrewbailey.com/ ), I have a "health check" page that includes all available trust chains. For my Let's Encrypt certificate, it shows 2: one through an intermediate to the DST Root, and another intermediate to the ISRG Root. I can verify that both exist and are used (though one certificate and intermediate are loaded and served): the current Firefox release (and all other browsers I've tried) uses the DST root, but Firefox Developer Edition uses the ISRG root.

It seems to make sense: certificates don't sign certificates, keys sign certificates (more specifically, certificate requests). I don't see an obvious reason that a single certificate request can't be signed more than once. If one is, trust should flow through either certificate, since the certificate says that the signer verified that the holder has the associated private key, and if that private key signed another certificate, it should not matter which intermediate the trust goes through, so long as the root on the other end is trusted.

[rocqua]

Thanks! I can't seem to find the direct 'health-check' page though.

Also, according to the spec, certificates sign a 'tbsCertificate' which contains all data of a certificate except for the actual signature and the field that determines what signing algorithm was used.

[theandrewbailey]

It's behind a login, but you can see the browser certificate chain in the browser UI itself. That should work for any Let's Encrypt certificate.