[dwaite]

There is a cross-device system to sign in, using QR and proximity checks.

Once the user has signed in, a modality check shows that they logged in with another device, while a capability check shows that they _could_ have authenticated with the local device if it had been registered. This may trigger the site to prompt them to register the local device as a second mechanism (or they may just go to the self-service account management tab to do it themselves).