ZeroSSL's current RSA intermediate is https://crt.sh/?id=2427368505, which chains up to USERTrust RSA Certification Authority (https://crt.sh/?caid=1167).

I was going to migrate over to ZeroSSL, but there were red flags in the form of missing documentation that you would expect from a CA, like what is the chain of trust for certificates that are being issued? If I have to issue myself a certificate to check which CA is being used to sign the cert, that doesn't feel right.

Relevant for Apple OSs: https://support.apple.com/en-us/HT209143

"Buypass Class 3 Root CA", which appears to be the root certificate they currently use, is present for all listed iOS versions (7+), which seems like a good sign. Let's Encrypt's "ISRG Root X1" is present in iOS 10+.

Similar lists for Android would be wonderful but probably impossible to compile due to ecosystem fragmentation. I guess there is no caniuse.com for root certificates.

I suspect there is no source that tracks exactly what's trusted on a large range of devices. Perhaps somebody should maintain this information, although it seems like a really thankless volunteer task, I'm really interested in such stuff and still it makes me feel tired just thinking about it.

Not sure its reliable but I've found this comparison: https://www.xf.is/2020/06/30/list-of-free-acme-ssl-providers...

Is there a range of trust? AFAIK you either trust a cert (directly or transitively) or not.