Rather than using autofill, why not create an open standard for a website to communicate with a password manager. In a similar way to OAuth (eg login with Google), sites could add a login with password manager button. Rather than being limited to a single provider, it would use whatever password manager add-on was installed. Benefits:
- Would avoid any cases where autofill fails
- Would simplify account creation (one click rather than having to both type a username and generate a password)
- Could replace the complexity of having multiple OAuth options with a single option
- Would allow for more easy adoption of more secure login methods - the password manager could decide what protocol or key length to use
- Is an open approach that would allow collaboration between different solutions in the space rather than fragmentation