I've created an Explainer here to help communicate more of the intent behind the proposed new error codes:

https://github.com/w3c/webauthn/wiki/Explainer:-New-Error-Codes-(2024-Edition)

Without judgement of the proposed errors, some spec formalia: currently, most WebAuthn errors are instances of DOMException with different names. The DOMException names table section of the WebIDL spec states that:

When creating or throwing a DOMException, specifications must use one of these names. If a specification author believes none of these names are a good fit for their case, they must file an issue to discuss adding a new name to the shared namespace [...]

So if we're going to continue using DOMException for all of these errors (I currently don't know any arguments for nor against it), we may need to pick from the already existing names, or generalize the error names enough that they can be upstreamed and useful in other contexts.

Fortunately, TimeoutError already exists, and OptOutError seems like a suitable match for UserCancelledError. The others are trickier to find existing matches for, though.

As summarized in #2047 (thanks!), the errors we already use are AbortError, ConstraintError, InvalidStateError, NotAllowedError, NotSupportedError, SecurityError and UnknownError (hm, turns out TypeError is not a DOMException but a simple exception). Adding TimeoutError and OptOutError to that, and removing deprecated names, the remaining ones are:

• DataCloneError: The object can not be cloned.
• DataError: Provided data is inadequate.
• EncodingError: The encoding operation (either encoded or decoding) failed.
• HierarchyRequestError: The operation would yield an incorrect node tree.
• InUseAttributeError: The attribute is in use by another element.
• InvalidCharacterError: The string contains invalid characters.
• InvalidModificationError: The object can not be modified in this way.
• InvalidNodeTypeError: The supplied node is incorrect or has an incorrect ancestor for this operation.
• NamespaceError: The operation is not allowed by Namespaces in XML.
• NetworkError: A network error occurred.
• NoModificationAllowedError: The object can not be modified.
• NotFoundError: The object can not be found here.
• NotReadableError: The I/O read operation failed.
• OperationError: The operation failed for an operation-specific reason.
• QuotaExceededError: The quota has been exceeded.
• ReadOnlyError: The mutating operation was attempted in a "readonly" transaction.
• SyntaxError: The string did not match the expected pattern.
• TransactionInactiveError: A request was placed against a transaction which is currently not active, or which is finished.
• VersionError: An attempt was made to open a database using a lower version than the existing version.
• WrongDocumentError: The object is in the wrong document.

I don't think any of these are good matches for HybridPrerequisitesError, UserHybridCancelError or UserVerificationError. The only somewhat-applicable one would be OperationError, but only because it's so unspecific it could apply to anything.

So maybe we need to use DOMException derived interfaces for these errors? Maybe a WebAuthenticationError with an enum-like DOMString cause attribute, for example.

Fortunately, TimeoutError already exists, and OptOutError seems like a suitable match for UserCancelledError. The others are trickier to find existing matches for, though.

As a note, we added OptOutError very recently for a specific use-case we had in Secure Payment Confirmation that didn't seem to be covered by existing DOMException types. That is, the WHATWG is open to adding new DOMException names, so please don't feel constricted by that!

Can we also add fine-grained errors in place of the NotSupportedError? These are the two scenarios I am thinking about:

1. Client doesn't support the requested extension
2. Client doesn't support the requested COSE algorithm

Finer grained error will help troubleshooting why a certain authenticator isn't working on RP side.

Client doesn't support the requested extension

Two things re: extensions:

1. Extensions are optional, so a WebAuthn ceremony will not fail because of an extension. If you request one and don't receive a response, it can be inferred that it isn't supported.
2. Extensions can be advertised via getClientCapabilities

I want to +1 this, given that we observe users often face unclear or overloaded errors, which limit their ability to troubleshoot and understand next steps. This is based on feedback we constantly receive about WebAuthn registrations. Errors like NotAllowedError are particularly challenging, as they obscure whether the issue is due to user cancellation, device compatibility, or other factors. Introducing more specific errors would provide users with clearer guidance and improve the experience across browsers and RPs.

To support users better on our side, we reviewed common WebAuthn errors and mapped likely causes with recommended actions to help users navigate these issues effectively. Providing context-specific messages, as seen with other RPs, helps users receive actionable guidance to complete or troubleshoot registration. Having this supported in the spec would be ideal.