The problem is, I don't want what you describe. I don't want my browser holding those credentials or passing them by proxy. It's too easily compromised or spoofed. I don't want the cloud holding those credentials, for the same reason. Regardless of what the credentials are, I want them either in my brain or in a passphrase-encrypted store which does not directly interface with the browser. So, your proposal needs a big "do it the old way" button.