There might actually be the beginnings of a de facto standard. Following the links in the recent posting Change password detection: is not Chromium forms compliant you can find this implementation note from the Chromium developers:
Password Form Styles that Chromium Understands
In my experience, Chromium (in its production form Chrome) often fails to autofill authentication information, and also often autofills the wrong information in the wrong place. So much work needs to be done to standardize logins.