I love that they did that; it was actually my idea (https://bugzilla.mozilla.org/show_bug.cgi?id=657228). I believe the list is pretty large and changes frequently and so they download it dynamically.
> short cut to a "Yes"
Do they really do that? That's awesome if so. Then they don't even need to ship the roots.
> I specifically don't like [...] saying "unknown issuer"
https://github.com/briansmith/webpki/issues/221
> If std::fs::File::open() gives me Result with an io:Error that claims "File not found" but the underlying OS file open actually failed due to a permission error, you can see why that's a problem right? Even if this hypothetical OS doesn't expose any specific errors, "File not found" is misleading.
A more accurate analogy: You ask to open "example.txt" without supplying the path, and there is no "example.txt" in the current working directory. You will get "file not found."
Regardless, I agree we could have a better name than UnknownIssuer for this error.