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The current text for exportKey() doesn't seem to provide for any errors being returned when exporting a private key in pkcs8 format. Whereas other methods like encrypt(), offer an escape hatch in the generic description saying that if an error occurs to return an OperationError. The NSS functions for exporting key allow for such errors so it may be possible for implementation based on it to fail here. Failures might also occur when exporting keys from secure elements.
https://dvcs.w3.org/hg/webcrypto-api/rev/3bc402e6c907