Add Cross Device Fallback URL extension for hybrid transport authentication

[harshlal028]

Closes #2341

The following tasks have been completed:

• Modified Web platform tests (link)

Implementation commitment:

• WebKit (link to issue)
• Chromium (link to issue)
• Gecko (link to issue)

Documentation and checks

• Affects privacy
• Affects security
• Updated explainer (link)

---

💥 Error: 422 Unprocessable Entity 💥

PR Preview failed to build. (Last tried on Jul 16, 2026, 3:55 AM UTC).

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PR Preview relies on a number of web services to run. There seems to be an issue with the following one:

🚨 Spec Generator - Spec Generator is the web service used to build bikeshed/ReSpec specs

🔗 Related URL

Error output:

[
    {
        "lineNum": "2067:21",
        "messageType": "fatal",
        "text": "Tried to auto-close a <li>, but there were unclosed elements remaining before the nearest matching start tag (at 1923:5).\nOpen tags: <div> at 1705:1, <li> at 1923:5, <dl> at 1926:9"
    },
    {
        "lineNum": "813:1",
        "messageType": "link",
        "text": "Multiple possible 'ctap2 canonical cbor encoding form' dfn refs.\nArbitrarily chose https://fidoalliance.org/specs/fido-v2.1-ps-20210615/fido-client-to-authenticator-protocol-v2.1-ps-errata-20220621.html#ctap2-canonical-cbor-encoding-form\nTo auto-select one of the following refs, insert one of these lines into a <pre class=link-defaults> block:\nspec:fido-client-to-authenticator-protocol-v2.1; type:dfn; text:ctap2 canonical cbor encoding form\nspec:fido-client-to-authenticator-protocol-v2.2; type:dfn; text:ctap2 canonical cbor encoding form\nspec:fido-client-to-authenticator-protocol-v2.3; type:dfn; text:ctap2 canonical cbor encoding form"
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        "text": "Multiple possible 'ctap2 canonical cbor encoding form' dfn refs.\nArbitrarily chose https://fidoalliance.org/specs/fido-v2.1-ps-20210615/fido-client-to-authenticator-protocol-v2.1-ps-errata-20220621.html#ctap2-canonical-cbor-encoding-form\nTo auto-select one of the following refs, insert one of these lines into a <pre class=link-defaults> block:\nspec:fido-client-to-authenticator-protocol-v2.1; type:dfn; text:ctap2 canonical cbor encoding form\nspec:fido-client-to-authenticator-protocol-v2.2; type:dfn; text:ctap2 canonical cbor encoding form\nspec:fido-client-to-authenticator-protocol-v2.3; type:dfn; text:ctap2 canonical cbor encoding form"
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        "text": "Multiple possible 'ctap2 canonical cbor encoding form' dfn refs.\nArbitrarily chose https://fidoalliance.org/specs/fido-v2.1-ps-20210615/fido-client-to-authenticator-protocol-v2.1-ps-errata-20220621.html#ctap2-canonical-cbor-encoding-form\nTo auto-select one of the following refs, insert one of these lines into a <pre class=link-defaults> block:\nspec:fido-client-to-authenticator-protocol-v2.1; type:dfn; text:ctap2 canonical cbor encoding form\nspec:fido-client-to-authenticator-protocol-v2.2; type:dfn; text:ctap2 canonical cbor encoding form\nspec:fido-client-to-authenticator-protocol-v2.3; type:dfn; text:ctap2 canonical cbor encoding form"
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[sbweeden]

Would like to see clarification on whether the fallbackURL can be server-relative or not.

[nsatragno]

This is not an authenticator step?

[harshlal028]

I grouped it under the current section as per (#2380 (comment)). As this is something that needs to be happen on authenticator device (by the hybrid implementation)- and there were only two options client and authenticator. Hence Is used the term remote client platform`. Lmk if you have a different suggestion and I can make the change.

[timcappalli]

@harshlal028 can you merge in main so that the PR preview starts working again?

[harshlal028]

@harshlal028 can you merge in main so that the PR preview starts working again?

Hi @timcappalli,
Here is the main branch. Git log after latest git pull

commit 3cf2143 (HEAD -> main, origin/main, origin/HEAD)
Merge: b668301 ddde943
Author: David Waite david@alkaline-solutions.com
Date: Wed Jan 7 15:26:43 2026 -0700

---

Here is my branch add-fallback-url-extension which is already on top of main. Git log from my branch

commit 5c15306
Author: Harsh Lal 32895139+harshlal028@users.noreply.github.com
Date: Mon Jan 26 18:42:14 2026 +0000

Add fallbackUrl client extension for hybrid transport authentication

commit 3cf2143 (origin/main, origin/HEAD, main)
Merge: b668301 ddde943
Author: David Waite david@alkaline-solutions.com
Date: Wed Jan 7 15:26:43 2026 -0700

Is there something else you would like me to do.

---

PS: I did build the spec using following commands and I am able to see the generated HTML with changes

$ python3 -m venv .python-venv
$ source .python-venv/bin/activate
$ pip install bikeshed==5.4.2
$ bikeshed spec