> In what universe is that simple? Your solution: browser extension +
> wallet + comminucation API + all the wallets need to implement it
> Our solution: just browser extension.

Browser extension would only be required until browsers add native support for detecting the tag and prompting a wallet client. This probably won't happen in the near future, though.

Also, the kind of browser extension you're talking about would be limited to just one device or require manually configured syncing between your devices, and would also likely be limited to just a few platforms.

The communication is done between the wallet and merchant the same as always with BIP70, but with some extra BIP70 extensions for this purpose. It just starts talking earlier.

It supports graceful degradation just fine, if the browser or wallet don't support it or the wallet isn't linked to that computer's browser, then nothing out of the ordinary happens. The browser extension really don't do anything special, it just relays the details in the HTML tag.

> > As one example, your browser could ask your hardware wallet over BLE for
> > this data. This way you barely have to trust the computer you're using at
> > all, as everything it does is confirmed on the hardware wallet before
> > payment (assuming it has a screen, which it should). Linking your hardware
> > wallet over BLE to new devices which you then use for browsing and shopping
> > could  be trivial and yet allow secure auto-fill of this kind.
>
> This looks more interesting but is information about your location
> really so secret that you need to hold it in HW wallet? Because if so,
> you probably don't want to use untrusted machine anyway. (Or just use
> Qubes OS.)

It isn't necessarily top secret, but why not be protective by default? Your hardware wallet is already designed to keep secrets. Lets say you're at a library computer, or at a friend's house, why not let your hardware wallet deal with all the security?

In this scenario it is likely already functioning as a central point for all your Bitcoin related purchases anyway, so it might as well be the device that remembers all your shopping preferences for you. So let's make it simple to use!