Yup! Both Microsoft and Gmail accounts will allow you to authenticate via a USB security key, which works with the Fido U2F app on my Ledger.

With Microsoft you only need to type your email and select "Sign in with USB key". With Gmail its not really a "primary" auth method but once you set up the security key in your Gmail account you can trigger it by clicking "Forgot your password", so you can either use it if you forget your password or as 2FA.

Surprisingly, Protonmail does not support this.

Really cool stuff, though.

🤣 how do I access this account from a different device? Say a desktop computer

I can. I'd just have to change how this is structured in the db. Currently, it's like

user {
   id
   name 
   ....
   pubkey
}

and I'd have to make it

user {
   id
   name 
   ....
}

pubkey {
   pubkey
   userId
}

and also rework the linking UI. Shouldn't take more than a couple hours to do though once started ... just that migrating the db needs to be done carefully.