The WebAuthn API and FIDO2/CTAP2 spec is supported on all major platforms/browsers now, including iOS, Android and Windows 10 (Hello). This means you can implement a similar experience to native iOS apps in a browser application if you wish as well as supporting FIDO compliant security keys also.
You can try it for yourself here: https://webauthn.io/
You'd need a backend that can support registration of credentials and authentication of assertions however. This is potentially non-trivial depending on what you have in play but library support is improving all the time.
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