Earlier quoted context omitted.
I was thinking of writing a spec and implementation on a similar thing a few days ago: Each site would publish a standardized file in a standardized location, and/or add a META tag to the file in each page. The file itself would simply contain the URL to the login page and the parameters that should be passed to it, and the way to pass those parameters. Then, your browser/password manager could simply perform a reque…
Unfortunately that requires mass standardization (adopting the spec), which is probably unlikely given that no one standardizes on the registration form to begin with.
The advantage, where password managers would just log you in transparently on any site, would hopefully be big and visible enough for more and more sites to start implementing this.